* upstream/main: (146 commits)
  Revert "[Native] Gradle-related tweaks to improve handling of the simdvec native library (elastic#144539)"
  Fix ArrayIndexOutOfBoundsException in fetch phase with partial results (elastic#144385)
  ESQL: Correctly manage NULL data type for SUM (elastic#144942)
  [ESQL] Fixes GroupedTopNBenchmark not executing (elastic#144944)
  Fix reader context leak when query response serialization fails (elastic#144708)
  Validate individual offset values in BULK_OFFSETS bounds checks (elastic#144643)
  Merge main21 source set into main in simdvec (elastic#144921)
  [TEST] Unmute TsidExtractingIdFieldMapperTests (elastic#144848)
  [Native] Gradle-related tweaks to improve handling of the simdvec native library (elastic#144539)
  Fix `ThreadedActionListenerTests#testRejectionHandling` (elastic#144795)
  Add new DLM Frozen Tier Transition execution plugin and service (elastic#144595)
  Prometheus: execute query_range via parsed EsqlStatement plan (elastic#144416)
  Investigate `testBulkIndexingRequestSplitting` failure (elastic#144766)
  Add test utility for wrapping directories in FilterDirectory layer (elastic#143563)
  Fix ES|QL decay tests with negative scale (elastic#144657)
  Fix circuit breaker leak in percolator query construction (elastic#144827)
  Use XPerFieldDocValuesFormat in AbstractTSDBSyntheticIdCodec (elastic#144744)
  [DOCS] Document how reindex work in CPS (elastic#144016)
  Fix Int4 vector library tests failing on Java 21 (elastic#144830)
  [DiskBBQ] Fix index sorting on flush (elastic#144938)
  ...

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.

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.