tweak ci workflow

.github/workflows/ci.yml

@@ -1,24 +1,32 @@
 name: CI
 
 on:
-  push:
-    branches: [ main ]
+  workflow_dispatch:
   pull_request:
-    branches: [ main ]
+    paths-ignore:
+      - '**/*.md'
+    branches:
+      - main
+  push:
+    paths-ignore:
+      - '**/*.md'
+    branches:
+      - main
 
 jobs:
   build:
     strategy:
       matrix:
-        java: [ '17' ]
-    runs-on: ubuntu-latest
-
+        java: [ '21' ]
+        os: [ 'ubuntu-latest' ]
+    runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v4
     - name: Set up JDK
       uses: actions/setup-java@v4
       with:
         java-version: ${{ matrix.java }}
         distribution: 'temurin'
-    - name: Run tests
-      run: mvn -B -ntp clean test
+        cache: 'maven'
+    - name: Build
+      run: mvn -ntp -B clean test