Add Java 11 to the Gradle workflow matrix

[wmdietl]

No description provided.

[Copilot]

Pull request overview

Updates the GitHub Actions Gradle CI workflow to include Java 11 in the test matrix, aligning CI with the project’s declared Java 11 source/target compatibility.

Changes:

• Add Java 11 to the Gradle workflow matrix.

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[Copilot]

The workflow matrix now includes Java 11, but the build/tests still won’t actually run on the matrix JDK because the job forces Gradle to use JDK 21 via org.gradle.java.home=$JAVA_HOME_21_X64 (so ./gradlew build, test, etc. run on 21 for every matrix entry). If the goal is to validate Java 11 compatibility, consider either (a) configuring Gradle toolchains / Test.javaLauncher to run tests on ${{ matrix.java }} while keeping the Gradle daemon on 21, or (b) adding an explicit runtime smoke test (run the built artifact with java from the matrix JDK) so each matrix entry provides real coverage.

[wmdietl]

@copilot Create a separate PR that does (a)

[Copilot]

@wmdietl I've opened a new pull request, #155, to work on those changes. Once the pull request is ready, I'll request review from you.