Limit permissions on GitHub actions.

Author: renggli

.github/workflows/maven.yml

@@ -1,11 +1,18 @@
 name: Java CI
+
 on:
   push:
     branches: [ main ]
   pull_request:
     branches: [ main ]
   schedule:
     - cron: "0 0 * * 0"
+  workflow_dispatch:
+defaults:
+  run:
+    shell: bash
+permissions:
+  contents: read
 
 jobs:
   build: