Improve label versions triaging

[picnixz]

Some people asked me about how I actually apply my labels while triaging and I think I could actually share my workflow in the devguide instead for future triaging. I especially think that it's not necessary for bugs that span across all active versions to be labelled with the all labels... Ideally, I would have wanted a 3.X+ tag which implies that the bug started appearing in 3.X, and a bot would be able to automate the labels every time a new one is added but this would be a separate issue.

cc @hugovk

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