Revert "Bump GitHub Actions JDK to 25"

This reverts commit a41e83e.

Author: eccentricdevotion

.github/workflows/gradle.yml

@@ -31,10 +31,10 @@ jobs:
         echo "$MULTI_LINE_CONTENT" >> $GITHUB_ENV
         echo "EOF" >> $GITHUB_ENV
 
-    - name: Set up JDK 25
+    - name: Set up JDK 21
       uses: actions/setup-java@v4
       with:
-        java-version: '25'
+        java-version: '21'
         distribution: 'temurin'
 
     # get short sha