operations: Enable use of detailed errors for UserMergeBranch

With UserMergeBranch we have introduced a first test balloon for the use of detailed gRPC errors which inject information about why specifically an RPC call has failed via embedded structured errors. While it has taken quite a while to roll this out, Rails has now been converted to use it. The result is that we have much more robust error handling, where Rails can now use the structured data to handle the error itself better without having to distinguish errors by matching messages.

This change has been rolled out to production on January 28th, and we didn't observe any issues with it. Remove the feature flag guarding the code.

Closes #3757 (closed)