CI build: upgrade from Java 18-EA to 18

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

.github/workflows/maven.yml

@@ -19,8 +19,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # TODO: switch from 18-ea to 18, as soon as Temurin offers the download on https://adoptium.net/releases.html
-        java: [ 11, 17, 18-ea ]
+        # Check for available Temurin releases on https://adoptium.net/releases.html
+        java: [ 11, 17, 18 ]
 
     runs-on: ubuntu-latest