Run builds only on push to master and pull requests

Author: garydgregory

.github/workflows/maven.yml

@@ -15,7 +15,11 @@
 
 name: Java CI
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - 'master'
+  pull_request: {}
 
 permissions:
   contents: read