chore: remove obsolete kokoro configs and update DocFX profile

[alicejli]

Partial fix for https://github.com/googleapis/google-http-java-client/issues/1933 (the other part of the fix will be in g3)

This removes the kokoro release configs as that was migrated to g3. This also updates the DocFX profile such that it javadoc generation will work. This repo doesn't inherit from java-shared-config AFAIK.

[generated-files-bot[bot]]

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

[alicejli]

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

[suztomo]

Do we use docFX for this repository?

[suztomo]

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