Ignore protobuf and types-protobuf in Dependabot

We're keeping our protobuf pin at 5 and shouldn't need to upgrade to
newer types-protobuf either, to match.

Author: stefanvanburen

.github/dependabot.yml

@@ -8,3 +8,7 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    ignore:
+      # We will manually update these as we drop compatibility for older versions.
+      - dependency-name: "protobuf"
+      - dependency-name: "types-protobuf"