Don't use Java 11 for community projects

There are compilation errors due to changes between 8 and 11

- addition of `String.lines` shadows the deprecated `StringOps.lines` in stdlib
  * probblem for ScalaTest
- JAXB APIs are removed
  * problem for BetterFiles

Author: liufengyun

.github/workflows/ci.yaml

@@ -101,9 +101,6 @@ jobs:
     container: lampepfl/dotty:2020-04-24
 
     steps:
-      - name: Set up JDK version
-        run: echo "::add-path::/usr/lib/jvm/java-11-openjdk-amd64/bin"
-
       - name: Checkout cleanup script
         uses: actions/checkout@v2