Install GraalVM JDK 21 on CI

Because that's what's used by the integration test

Author: marcphilipp

.github/workflows/main.yml

@@ -29,7 +29,7 @@ jobs:
       with:
         distribution: graalvm-community
         version: 'latest'
-        java-version: '24'
+        java-version: '21'
         github-token: ${{ secrets.GITHUB_TOKEN }}
     - name: Build
       uses: ./.github/actions/main-build