chore: disable renovate from updating gRPC in workspace files (#16406)

Author: scotthart

.github/renovate.json

@@ -39,6 +39,11 @@
       ],
       "enabled": false
     },
+    {
+      "matchFileNames": ["bazel/workspace0.bzl"],
+      "matchPackageNames": ["com_github_grpc_grpc", "grpc/grpc", "grpc"],
+      "enabled": false
+    },
     {
       "matchPackageNames": ["com_google_absl", "abseil/abseil-cpp", "abseil-cpp"],
       "groupName": "Abseil",