Restrict builds triggered by push to master

In the current configuration, builds run on all branches, including
those that are used to submit PRs.

For those branches the build workflow runs twice: once triggered
by `push` and once triggered by `pull_request`.

Author: ppkarwasz

.github/workflows/maven.yml

@@ -18,6 +18,8 @@ name: Java CI
 on:
   workflow_dispatch:
   push:
+    branches:
+      - 'master'
     paths-ignore:
       - '**/workflows/*.yml'
       - '!**/workflows/maven.yml'