[hotfix] notify maintainers on the diagnostics channel

• we used to notify ourselves on BOOST IT, now we tend to get notifications on DIAGNOSTICS. let's be consistent about that!
• while at it: send these notifications in sequence. if a big channel is destroyed or recycled, we don't want signald to shit the bed
• aaaand... let's make _notifyMany handle either a concrete message or a notification key so we can use it as a helper for all of our notify functions

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cc @mari