Clarify MR backwards compatibility
Recently, a combined backend/frontend MR !57452 (merged) introduced a field to the GraphQL schema. It was deployed in such a way that the frontend on Canary (with the new field available) was calling an old version of the backend GraphQL API (without the field available), causing an error for some users including @fguibert and @nagyv-gitlab.
https://docs.gitlab.com/ee/development/multi_version_compatibility.html#sidebar-wasnt-loading-for-some-users is the closest example to this, but it's not clear what should have been done in this case.
It seems that there should be explicit backwards-compatibility of both sides of this MR - catching errors in case myReactionEmoji can't be found, for example.
Proposal
Let's update these docs and make it clear for the best way to have handled this example.
We should modify the MR template so the docs regarding multi-version compatibility are easy to find and in the most relevant context.