GitHub Actions: check out proto submodule.

Author: UweTrottmann

.github/workflows/gradle.yml

@@ -11,6 +11,8 @@ jobs:
 
     steps:
       - uses: actions/checkout@v3
+        with:
+          submodules: true # For endpoints-management-protos/src/main/proto
 
       - name: Set up JDK 11
         uses: actions/setup-java@v3