Upgrade to reference actions/setup-java@v2

- Update all references in README.md
- Update version used in `pr-build-scan-comment`
- Discourage the use of `cache: gradle` with `setup-java@v2`

.github/workflows/pr-build-scan-comment.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - uses: actions/setup-java@v1
+    - uses: actions/setup-java@v2
       with:
         java-version: 11
     - uses: ./