Use Java 21 in the release workflow

Author: nakamura-to

.github/workflows/release.yml

@@ -37,11 +37,11 @@ jobs:
         if: github.event.inputs.version == null
         run: echo "RELEASE_VERSION=${{ steps.candidate-version.outputs.result }}" >> $GITHUB_ENV
 
-      - name: Set up JDK 8
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
-          java-version: 8
+          java-version: 21
 
       - name: Checkout
         uses: actions/checkout@v4