[actions] Drop jdk 18, add 20-ea, and bump 19 to ga

Author: hazendaz

Diff for: .github/workflows/ci.yaml

@@ -24,7 +24,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        java: [11, 17, 18, 19-ea]
+        java: [11, 17, 19, 20-ea]
         distribution: ['zulu']
       fail-fast: false
       max-parallel: 4