chore: update release CI to use Java 21 (#681)

Update both release.yml and snapshot.yml workflows to use Java 21 instead of Java 11 for building releases and snapshots.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: xerial

.github/workflows/release.yml

@@ -16,11 +16,11 @@ jobs:
           fetch-depth: 10000
       # Fetch all tags so that sbt-dynver can find the previous release version
       - run: git fetch --tags -f
-      # Install OpenJDK 11
+      # Install OpenJDK 21
       - uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
-          java-version: '11'
+          java-version: '21'
       - name: Setup GPG
         env:
           PGP_SECRET: ${{ secrets.PGP_SECRET }}

.github/workflows/snapshot.yml

@@ -27,7 +27,7 @@ jobs:
       - uses: actions/setup-java@v4
         with:
           distribution: 'zulu'
-          java-version: '11'
+          java-version: '21'
       - uses: actions/cache@v4
         with:
           path: ~/.cache