Fix CI workflow to use Maven instead of Gradle

Author: google-labs-jules[bot]

.github/workflows/build.yml

@@ -17,9 +17,5 @@ jobs:
         with:
           java-version: '17'
           distribution: 'temurin'
-      - name: Validate Gradle wrapper
-        uses: gradle/wrapper-validation-action@e6e38bacfdf1a337459f332974bb2327a31aaf4b
-      - name: Build with Gradle
-        uses: gradle/gradle-build-action@67421db6bd0bf253fb4bd25b31ebb98943c375e1
-        with:
-          arguments: clean build
+      - name: Build with Maven
+        run: mvn -B package --file pom.xml