Release CI: Bulletproof SemVer Checks

[ggwpez]

The current CI checks are structured in a way that the R0-silent label skips the SemVer checks. This is good in general, but i already saw two merge requests that declared R0 and had accidental breaking changes.

I think we should do something like this:

graph TD
    A(Merge Request) --> B{SemVer Change Detected?}
    B -- No --> Advise{Targets Stable?}
    Advise -- No --> Ok
    Advise -- Yes --> Comm[Comment Generic backport advise]
    B -- Yes --> R1{Has R0 label?}
    R1 -- No --> S{Targets Stable?}
    S -- No --> Ok2[Check PrDocs]
    S -- Yes --> nM[Ensure PrDocs are minor]
    R1 -- Yes --> Major{Is Major change?}
    Major -- No --> W[Warn about removing R0]
    Major -- Yes --> Stable{Targets Stable?}
    Stable -- No --> W
    Stable -- Yes --> E[Error: Forbidden]

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