Install Maven 3.8.8 explicitly to avoid Java 17 requirement

Maven 3.9.0+ enforces stricter Java prerequisites for plugins.
The quarkus-maven-plugin:3.6.9 requires Java 17 to execute when
used with Maven 3.9.x, even though it's compiled for Java 11.

Maven 3.8.8 (last pre-3.9 version) doesn't enforce this check,
allowing the build to work with Java 11.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: ruhan1

.github/workflows/merge-build.yml

@@ -44,6 +44,11 @@ jobs:
         architecture: x64
         java-version: 11
 
+    - name: Set up Maven
+      uses: stCarolas/setup-maven@v4.5
+      with:
+        maven-version: 3.8.8
+
     - name: maven-settings-xml-action
       uses: whelk-io/maven-settings-xml-action@v14
       if: ${{ github.event.repository.fork == false }}

.github/workflows/pr-build.yml

@@ -35,6 +35,11 @@ jobs:
         java-version: '11'
         distribution: 'adopt'
 
+    - name: Set up Maven
+      uses: stCarolas/setup-maven@v4.5
+      with:
+        maven-version: 3.8.8
+
     - name: maven-settings-xml-action
       uses: whelk-io/maven-settings-xml-action@v14
       with: