Add deprecations to PR summary

Recently we have received at lot of PRs (especially in the Form component) that have a `Backwards compatibility break: no` only because they provide a "legacy layer" which is intended to be deprecated in 2.3 or 3.0.

This means that merging those PRs will ultimately cause BCs and that there is no other way than looking at the code in order to know what they will be. I think this is not fair.

Adding the `deprecations` header would allow indicating those BC breaks in advance and help users sreach the issue tracker when they will hit errors.

/cc @bschussek