Fix SpotBugs Java version - run with JDK 11

SpotBugs 4.9.2 requires Java 11+ (class file version 55.0) but the
workflow was running it with Java 8. Updated workflow to use JDK_11
environment variable for SpotBugs tasks.

Co-authored-by: torosent <[email protected]>

Author: Copilot

.github/workflows/build-validation.yml

@@ -45,7 +45,9 @@ jobs:
       run: ./gradlew build -x test
 
     - name: Run SpotBugs
-      run: ./gradlew spotbugsMain spotbugsTest
+      run: |
+        export JAVA_HOME=$JDK_11
+        ./gradlew spotbugsMain spotbugsTest
       continue-on-error: false
 
     - name: Upload SpotBugs reports