Prepend -Xlint:none before -Xlint:all,... in ElasticsearchJavaBasePlugin
to suppress the mandatory "using incubating module(s)" warning that javac
emits when jdk.incubator.vector is used. On JDK 21 this warning has no
named lint category and cannot be individually suppressed (-Xlint:-incubating
was added in JDK 22). -Xlint:none clears all warnings including this
mandatory one, then -Xlint:all,... re-enables only the named categories.

For JDK 22+ compilers, -incubating is also added to the exclusion list
to handle JDK 25+ where -Xlint:all re-enables the now-named category.

This allows removing the -Werror workarounds from simdvec and swisshash
build.gradle files, restoring warnings-as-errors for all source files.

This is step 2 of #144797. I'm merging the `main21` source set into `main` in the simdvec module, eliminating one level of MR-JAR complexity.

The `main21` source set existed because it uses `jdk.incubator.vector`, which emitted an unsuppressible warning on JDK 21 that was incompatible with `-Werror`. Now that I've suppressed that warning centrally in #144798, there is no longer a reason to keep these files in a separate source set.

Specifically:
  - I moved all 33 source files from `src/main21/java/` into `src/main/java/`.
  - I replaced the two stub classes in `main` (`VectorScorerFactoryImpl`,
    `ESVectorizationProvider`) with their real `main21` implementations. The
    stubs only existed to satisfy the compiler and threw
    `UnsupportedOperationException` at runtime.
  - I moved `IndexInputUtilsTests` from `src/test21/` into `src/test/`.
  - I deleted the `src/main21/` and `src/test21/` directories entirely.
  - I updated `build.gradle` to pass `--add-modules` and `--add-reads` to the
    main `compileJava` task instead of `compileMain21Java`.
  - I updated the `module-info.java` Javadoc to reflect the simplified layout.
  
  I've left the `main22` source set and the MR-JAR plugin in place for now;
  I'll address those in subsequent steps.

This is step 2 of elastic#144797. I'm merging the `main21` source set into `main` in the simdvec module, eliminating one level of MR-JAR complexity.

The `main21` source set existed because it uses `jdk.incubator.vector`, which emitted an unsuppressible warning on JDK 21 that was incompatible with `-Werror`. Now that I've suppressed that warning centrally in elastic#144798, there is no longer a reason to keep these files in a separate source set.

Specifically:
  - I moved all 33 source files from `src/main21/java/` into `src/main/java/`.
  - I replaced the two stub classes in `main` (`VectorScorerFactoryImpl`,
    `ESVectorizationProvider`) with their real `main21` implementations. The
    stubs only existed to satisfy the compiler and threw
    `UnsupportedOperationException` at runtime.
  - I moved `IndexInputUtilsTests` from `src/test21/` into `src/test/`.
  - I deleted the `src/main21/` and `src/test21/` directories entirely.
  - I updated `build.gradle` to pass `--add-modules` and `--add-reads` to the
    main `compileJava` task instead of `compileMain21Java`.
  - I updated the `module-info.java` Javadoc to reflect the simplified layout.
  
  I've left the `main22` source set and the MR-JAR plugin in place for now;
  I'll address those in subsequent steps.

Prepend -Xlint:none before -Xlint:all,... in ElasticsearchJavaBasePlugin
to suppress the mandatory "using incubating module(s)" warning that javac
emits when jdk.incubator.vector is used. On JDK 21 this warning has no
named lint category and cannot be individually suppressed (-Xlint:-incubating
was added in JDK 22). -Xlint:none clears all warnings including this
mandatory one, then -Xlint:all,... re-enables only the named categories.

For JDK 22+ compilers, -incubating is also added to the exclusion list
to handle JDK 25+ where -Xlint:all re-enables the now-named category.

This allows removing the -Werror workarounds from simdvec and swisshash
build.gradle files, restoring warnings-as-errors for all source files.

This is step 2 of elastic#144797. I'm merging the `main21` source set into `main` in the simdvec module, eliminating one level of MR-JAR complexity.

The `main21` source set existed because it uses `jdk.incubator.vector`, which emitted an unsuppressible warning on JDK 21 that was incompatible with `-Werror`. Now that I've suppressed that warning centrally in elastic#144798, there is no longer a reason to keep these files in a separate source set.

Specifically:
  - I moved all 33 source files from `src/main21/java/` into `src/main/java/`.
  - I replaced the two stub classes in `main` (`VectorScorerFactoryImpl`,
    `ESVectorizationProvider`) with their real `main21` implementations. The
    stubs only existed to satisfy the compiler and threw
    `UnsupportedOperationException` at runtime.
  - I moved `IndexInputUtilsTests` from `src/test21/` into `src/test/`.
  - I deleted the `src/main21/` and `src/test21/` directories entirely.
  - I updated `build.gradle` to pass `--add-modules` and `--add-reads` to the
    main `compileJava` task instead of `compileMain21Java`.
  - I updated the `module-info.java` Javadoc to reflect the simplified layout.
  
  I've left the `main22` source set and the MR-JAR plugin in place for now;
  I'll address those in subsequent steps.

This is step 2 of elastic#144797. I'm merging the `main21` source set into `main` in the simdvec module, eliminating one level of MR-JAR complexity.

The `main21` source set existed because it uses `jdk.incubator.vector`, which emitted an unsuppressible warning on JDK 21 that was incompatible with `-Werror`. Now that I've suppressed that warning centrally in elastic#144798, there is no longer a reason to keep these files in a separate source set.

Specifically:
  - I moved all 33 source files from `src/main21/java/` into `src/main/java/`.
  - I replaced the two stub classes in `main` (`VectorScorerFactoryImpl`,
    `ESVectorizationProvider`) with their real `main21` implementations. The
    stubs only existed to satisfy the compiler and threw
    `UnsupportedOperationException` at runtime.
  - I moved `IndexInputUtilsTests` from `src/test21/` into `src/test/`.
  - I deleted the `src/main21/` and `src/test21/` directories entirely.
  - I updated `build.gradle` to pass `--add-modules` and `--add-reads` to the
    main `compileJava` task instead of `compileMain21Java`.
  - I updated the `module-info.java` Javadoc to reflect the simplified layout.
  
  I've left the `main22` source set and the MR-JAR plugin in place for now;
  I'll address those in subsequent steps.