Drop jdk 18, add 20-ea, and update 19 to ga

Author: kazuki43zoo

.github/workflows/ci.yaml

@@ -28,7 +28,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macOS-latest, windows-latest]
-        java: [11, 17, 18, 19-ea]
+        java: [11, 17, 19, 20-ea]
         distribution: ['zulu']
       fail-fast: false
       max-parallel: 5