Add automation which warns PR authors when changes to upstream release branches will cause merge conflicts

[RikkiGibson]

Scenario:

• User opens a PR to release/dev16.8
• A bot attempts to merge those changes to master behind the scenes (possibly to further downstream branches, but it seems harder to reason about the cause and effect in that case)
• If there are conflicts in the merge, the bot leaves a comment with the conflict patch in there somehow to give the author a heads up that there will be conflicts in downstream releases after merging.

Alternatively, we may wish to reconsider the strategy of automatically flowing changes to downstream releases (as opposed to flowing to -vs-deps branches), in order to put the onus of making those changes work in other releases onto the IC and not onto our automated systems. This is what dotnet/runtime does, for example, and they have a handy "backport this to upstream release" automation that they can run on downstream PRs. e.g. dotnet/runtime#44477