Test JDK 21, 25 in Linux workflow

[wing328]

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

---

Summary by cubic

Expand the Linux CI matrix to run builds on JDK 21 and 25 (alongside 11 and 17). Remove Kotlin dynamic-compile test utilities and comment out related or unstable tests to keep CI stable on newer JDKs.

• Refactors
  • Delete KotlinTestUtils.java and KotlinTestUtilsTest.java.
  • Comment out dependent/unstable tests in KotlinClientCodegenModelTest, KotlinServerCodegenTest, and KotlinSpringServerCodegenTest (dollar-sign escaping, validation message assertions, and embedded enum array).

^{Written for commit e83ea47. Summary will update on new commits.}