Add TODO to maven.yml for switch to JDK 18 final

TODO: switch from 18-ea to 18, as soon as Temurin offers the download
on https://adoptium.net/releases.html.

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

.github/workflows/maven.yml

@@ -19,6 +19,7 @@ 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 ]
 
     runs-on: ubuntu-latest