tests: Remove 3.25.8 from protobuf compatibility testing (googleapis#4101)

Remove 3.25.8 from protobuf compatibility testing because the gen code
has already been upgraded to 4.x.

Author: blakeli0

.github/workflows/downstream_protobuf_compatibility_check_nightly.yaml

@@ -41,7 +41,7 @@ jobs:
           - java-storage-nio
         # Default Protobuf-Java versions to use are specified here. Without this, the nightly workflow won't know
         # which values to use and would resolve to ''.
-        protobuf-version: ${{ fromJSON(format('[{0}]', inputs.protobuf_runtime_versions || '"3.25.8","4.33.4"')) }}
+        protobuf-version: ${{ fromJSON(format('[{0}]', inputs.protobuf_runtime_versions || '"4.33.5"')) }}
     steps:
       - name: Checkout sdk-platform-java repo
         uses: actions/checkout@v4