Deprecate RPC UserRebase

We are deprecating the UserRebase RPC in favour of a new Rebase RPC that can yield the rebase commit SHA to the client before continuing with the rebase. The reason for this new functionality is outlined in this issue comment https://gitlab.com/gitlab-org/gitlab-ee/issues/5966#note_150894306.

Gitaly MR !1208 (merged)
gitaly-proto MR gitaly-proto!293 (merged)

put a deprecation comment // DEPRECATED: <ISSUE-LINK> in gitaly-proto MR link: gitaly-proto!293 (merged)
update GitLab-Rails client-side uses to no longer use RPC: gitlab!23546 (merged), gitlab!24785 (merged)
wait for a GitLab to release ** 12.8 **
delete the server side implementation of the old RPC in https://gitlab.com/gitlab-org/gitaly Merge Request LINK

Edited Feb 19, 2020 by Zeger-Jan van de Weg

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