MAINTAINERS.md: add emails and PGP keys

[ulysses4ever]

this fixes #11245

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Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

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• Patches conform to the coding conventions.
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[ffaf1]

Very good.

Matthew and Artem keys lack cross signatures, which I plan to rectify.

Matthew email is not present on his key, which is fine but let me ping him to make 100% sure we got the correct key.

[ulysses4ever]

thank you Francesco, I hoped that someone would assess these critically since I just took what I could quickly google for. Matt's key is especially suspicious I agree.

[ffaf1]

Forgot the ping, @mpickering

[mpickering]

As far as I'm aware, I don't have a PGP key.

[ulysses4ever]

all right, we can skip the key for Matthew for now, maybe?

[geekosaur]

You probably don't want to use fixups any more unless you squash manually: Mergify is dropping support for autosquash.

[ulysses4ever]

@geekosaur we've discussed it before (maybe several times): I use fixups not because of support from Mergify but because it's how I'm used to format fixup commits.