Run build only on master and PRs

When a PR is created from a branch in the same repository, workflows run twice: once on `push` and once on `pull_request`.

This change fixes it, by only running workflows:

- On a direct push to `master`,
- On a Pull Request, regardless of the origin.

Author: ppkarwasz

.github/workflows/maven.yml

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