Upgrade to Java 21

[vasiliy-mikhailov]

Upgrades the build from Java 17 to Java 21 (current LTS).

• pom.xml: maven-compiler-plugin <release> 17 → 21
• .github/workflows/test.yml & release.yml: java-version 17 → 21, so CI builds on the new target (otherwise the JDK-17 runners can't compile a release-21 build)

Verification: the project compiles under JDK 21 and all 162 unit tests pass, with none lost versus the JDK-17 baseline. Integration tests that need external services (MongoDB/MariaDB/Docker) were not run locally and are unchanged. No production or test code was modified — only the compiler target and the CI JDK.

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How this was produced

The compiler-target change was produced with OpenRewrite's UpgradeJavaVersion recipe:

# rewrite.yml
type: specs.openrewrite.org/v1beta/recipe
name: com.example.UpgradeJava
recipeList:
  - org.openrewrite.java.migrate.UpgradeJavaVersion:
      version: 21

mvn -U org.openrewrite.maven:rewrite-maven-plugin:RELEASE:run \
  -Drewrite.configLocation=$(pwd)/rewrite.yml \
  -Drewrite.activeRecipes=com.example.UpgradeJava \
  -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-migrate-java:RELEASE

The diff was then reduced by hand to the minimal compiler-target change, and the CI workflow java-version was updated to match (OpenRewrite does not edit GitHub Actions).

Verified: all 162 tests pass under JDK 21, none lost versus the baseline.

[vasiliy-mikhailov]

Closing this out — on reflection a standalone Java-version bump isn't really worth your review time without a clear benefit, and I didn't want to leave it sitting in your queue. Thanks for the work on this project.