Removal of `RPCCredentials` as it was completely the same
implementation as `RPCCredentialsV2`.
Fix all places where `RPCCredentials` was used.
Removal of `v1` token representation check and validation.
Closes: https://gitlab.com/gitlab-org/gitaly/-/issues/2498
This change swaps V1 authentication with V2 without the caller knowing
it did. This allows for a smooth transition toward V2.
Idea taken from: https://gitlab.com/gitlab-org/gitaly/issues/2498#note_293995290
Leveraging `go fix` these changes were made, I only applied some regexp
to get the import grouping correct.
The old package can't be removed from vendor directory yet, as other
dependencies depend on it.