Regenerate Java client and release v2.0.0

[kwent]

Summary

• regenerate the Java client from the current Rootly OpenAPI specification
• fix regeneration so stale generated tests are removed before generation
• prepare the breaking v2.0.0 package version
• add Release Drafter with OpenAPI changelog assets and tag-push publication
• migrate all GitHub Actions jobs to Blacksmith Ubuntu 24.04 runners
• test Java 17, 21, and 25 in Maven CI

Breaking changes

Compared with v1.0.0, oasdiff reports 486 breaking errors and 443 potentially breaking warnings. Examples include removed request enum values, newly required request properties, and response properties becoming optional or nullable. This requires a major release.

Validation

• make test: passed
• Maven package on Java 17: passed (9,545 tests, 0 failures, 0 errors, 589 generated API tests skipped)
• GitHub workflow YAML parsed successfully

Release

After merge, pushing the v2.0.0 tag will cause Release Drafter to publish the GitHub release and the publish workflow to deploy Maven Central and GitHub Packages artifacts.