update pipeline

Author: Andreas Dann

.github/workflows/maven-deploy.yml

@@ -2,6 +2,8 @@ name: Publish package to GitHub Packages and attach to release
 on:
   release:
     types: [created]
+    branches:
+      - main
 
 # if pushing docker images fails https://stackoverflow.com/a/71193319 -- make sure to allow access for GitHub Actions to that package registry
 env: