Find additional heuristics to source labels for the componentspecific model

[marco-c]

I couldn't think of other ways to find labels.
We can't rely on bugs that are currently in Firefox::General because some of them might be wrong. Even the fixed ones could have been wrong.

Originally posted by @marco-c in #5892 (comment)

[marco-c]

Maybe we could pick a subset that we think should be really good, e.g. uplifted bugs. But I would do this as a follow-up because it needs to be evaluated (and maybe we want to use them for training but not for testing, which means we must wait for #4407).

We could later use the changed files in the linked patch could as heuristic (e.g., utilizing mots).