Warning: This pull request is touching the following templated files:

• .github/workflows/ci.yaml
• .kokoro/dependencies.sh
• .kokoro/release/bump_snapshot.cfg
• .kokoro/release/common.cfg
• .kokoro/release/common.sh
• .kokoro/release/drop.cfg
• .kokoro/release/drop.sh
• .kokoro/release/promote.cfg
• .kokoro/release/promote.sh
• .kokoro/release/publish_javadoc.cfg
• .kokoro/release/publish_javadoc.sh
• .kokoro/release/publish_javadoc11.cfg
• .kokoro/release/publish_javadoc11.sh
• .kokoro/release/snapshot.cfg
• .kokoro/release/snapshot.sh
• .kokoro/release/stage.cfg
• .kokoro/release/stage.sh

We can't add Java 21 to the series of tests due to compilation error:

Error:  Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.11.0:compile (default-compile) on project google-http-client: Compilation failure: Compilation failure: 
Error:  Source option 7 is no longer supported. Use 8 or later.
Error:  Target option 7 is no longer supported. Use 8 or later.
Error:  -> [Help 1]

That's the reasoning behind the custom test that compiles with 8 and then tests with 21: https://github.com/googleapis/google-http-java-client/blob/main/.github/workflows/ci.yaml#L90

Do we use docFX for this repository?

Never mind. Found it https://cloud.google.com/java/docs/reference/google-http-client/latest/overview.