Bump google-protobuf to 3.25.3

This was removed from the Gemfile.lock during [the Ruby 3.3
update](9a231fe)
but not removed as an actual dependency, therefore any subsequent runs
of `bundle install` have resulted in the incompatible version of the gem
being reinstalled.

Therefore upgrading to a version of `google-protobuf` that is compatible
with Ruby 3.3.

Gemfile.lock

@@ -152,7 +152,10 @@ GEM
       warden-oauth2 (~> 0.0.1)
     globalid (1.2.1)
       activesupport (>= 6.1)
-    google-protobuf (3.25.2)
+    google-protobuf (3.25.3)
+    google-protobuf (3.25.3-aarch64-linux)
+    google-protobuf (3.25.3-arm64-darwin)
+    google-protobuf (3.25.3-x86_64-linux)
     googleapis-common-protos-types (1.11.0)
       google-protobuf (~> 3.18)
     govuk_app_config (9.9.1)