Do not test against Java 19 & 20

Because Gradle 7.4 does not support them.

Author: acogoluegnes

.github/workflows/test-supported-java-versions.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ '8', '11', '17', '19', '20-ea' ]
+        java: [ '8', '11', '17' ]
     name:  Test against Java ${{ matrix.java }}
     services:
       rabbitmq: