Fix CI : revient sur ubuntu-latest, installe Maven via apt-get

Le container maven:3.9-eclipse-temurin-17 n'a pas Node.js, requis par
les actions JS (checkout, cache, upload-artifact) -> echec au premier
step. Retour au runner par defaut + installation explicite de Maven
(actions/setup-java installe le JDK mais jamais Maven).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -9,18 +9,20 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
-    container: maven:3.9-eclipse-temurin-17
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Cache Maven dependencies
-        uses: actions/cache@v4
+      - name: Setup JDK 17
+        uses: actions/setup-java@v4
         with:
-          path: ~/.m2/repository
-          key: ${{ runner.os }}-maven-${{ hashFiles('**/pom.xml') }}
-          restore-keys: |
-            ${{ runner.os }}-maven-
+          distribution: temurin
+          java-version: '17'
+          cache: maven
+
+      - name: Install Maven
+        run: |
+          which mvn || (sudo apt-get update && sudo apt-get install -y maven)
 
       - name: Build and test
         run: mvn -B clean verify