featureflag: Default-enable GoUpdateRemoteMirror
The Go implementation of UpdateRemoteMirror is currently hidden behind a feature flag. Given that its implementation has been rolled out to proudction on June 15th without any further issues, let's default-enable the feature flag.
This also fixes an issue we have with the rollout of in-memory remotes in Rails: they're only implemented in the Go port, but due to an oversight we already default-enabled usage of in-memory remotes in Rails. As a result, if the GoUpdateRemoteMirror feature flag wasn't manually enabled, Rails will now always fail.
Part of #3522 (closed)