Update GitHub Action from Java 14 to 15

Signed-off-by: Lars Grefer <eclipse@larsgrefer.de>

.github/workflows/maven.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java: [ 8, 11, 14 ]
+        java: [ 8, 11, 15 ]
     steps:
       - uses: actions/checkout@v2