Mark breaking changes that require user action

[sebproell]

In a discussion initiated by @ischeider, we (@georghammerl @mayrmt) came up with the idea to mark PRs that break input files with a new label. I just created this label "breaking change" ( see breaking change Pull requests that require manual user actions after merging ). We should now try to send a message to the general channel when such a PR is merged.

[mayrmt]

@sebproell Should we limit this to input files? There are also other breaking changes, e.g. updates to compilers. They are less frequent, but of similar severity. We could either do just a "breaking change" label, which might be easier than having different types of breaking change labels.

[sebproell]

I briefly thought about it, and mainly wanted to exclude all the "breaking changes" that we have inside the code, so I opted for the additional "input" tag. We do not care about internal breaks that much, since we do no provide a stable API to anything (yet) and are essentially a monorepo. So yes, just "breaking change" might also be clear enough. And yes, we could then also apply it, when dropping features that require user intervention (e.g. an ancient dependency version, a new CMake update).

edit: Changed label to "breaking change"