change from ruby setup to java setup in workflow gem-push

Author: chikamura

.github/workflows/main.yml

@@ -56,10 +56,10 @@ jobs:
     if: ${{ github.event_name == 'workflow_dispatch' || contains(github.ref, 'refs/tags/') }}
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Ruby 2.7
-        uses: ruby/setup-ruby@v1
+      - name: Set up JDK 1.8
+        uses: actions/setup-java@v1
         with:
-          ruby-version: 2.7
+          java-version: 1.8
       - name: push gem
         uses: trocco-io/push-gem-to-gpr-action@v1
         with: