Update maven.yml (exclude projectforge-webapp)

projectforge-webapp (build of react webapp) fails time to time. So skip it here.

Author: kreinhard

.github/workflows/maven.yml

@@ -25,5 +25,5 @@ jobs:
       with:
         java-version: '11'
         distribution: 'adopt'
-    - name: Build with Maven
-      run: mvn -B package --file pom.xml
+    - name: Build with Maven (excluding projectforge-webapp)
+      run: mvn -B package --file pom.xml -pl '!projectforge-webapp'