openapi-generator-gradle-plugin: upgrade Gradle to v8

[dennisameling]

The openapi-generator-gradle-plugin is currently built for Gradle 7.6.4, which is no longer supported. It also doesn't support Java 21. The currently supported versions are 8.x and 9.x.

Looking at the compatibility matrix, Gradle 8.14.x is compatible with Java 8 through 24, meaning that this upgrade should be non-breaking for users of this plugin. 7.6.x supported up until Java 19, so this PR will add support for Java 20 through 24 as well.

Lastly, this PR fixes some deprecations that were introduced in Gradle v8, like buildDir.

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.

Adding the Java and Kotlin technical committees: @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03) @stefankoppier (2022/06) @e5l (2024/10)

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Summary by cubic

Upgrade the openapi-generator-gradle-plugin build to Gradle 8.14.4 to add Java 20–24 support and resolve Gradle 8 deprecations without breaking existing users. CI now runs on JDK 21.

• Dependencies

  • Gradle wrapper and pom gradleVersion to 8.14.4; regenerate wrapper scripts and enable validateDistributionUrl.
  • Kotlin plugins to org.gradle.kotlin.kotlin-dsl 5.2.0 and kotlin-jvm 2.0.21; plugin-publish and nexus publish plugins to 2.0.0.
  • GitHub Actions uses Temurin JDK 21; sample project wrappers updated.

• Refactors

  • Replace $buildDir with layout.buildDirectory across plugin defaults, README, and samples to address Gradle 8 deprecations.
  • Update signing.required to assignment style; minor Kotlin test tweaks (use createTempDirectory).
  • Test matrix targets Gradle 8.14.4 and 8.5.

^{Written for commit d682e53. Summary will update on new commits.}

[cubic-dev-ai]

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