fix(ci): bump verifyWrapper JDK to 21 on 8.0.x

The verifyWrapper job pinned actions/setup-java to JDK 17, which works on 7.0.x and 7.1.x where javaVersion=17. When that workflow merged into 8.0.x (javaVersion=21), grails-wrapper's Java sources started being compiled with --release=21 (class file version 65), so the JDK 17 runtime in the verify step failed with: UnsupportedClassVersionError: grails/init/Start has been compiled by a more recent version of the Java Runtime (class file version 65.0), this version of the Java Runtime only recognizes class file versions up to 61.0.

Bump the verify step to JDK 21 to match the branch's javaVersion. Add an inline comment explaining the dependency so future bumps to javaVersion know to update this together.

Assisted-by: claude-code:claude-opus-4

Author: jamesfredley

.github/workflows/gradle.yml

@@ -506,7 +506,7 @@ jobs:
         uses: actions/setup-java@v4
         with:
           distribution: liberica
-          java-version: 17
+          java-version: 21
       - name: "📥 Download Wrapper"
         uses: actions/download-artifact@v7.0.0
         with: