Rename 'Trusted Users' to 'Package Maintainers'

Hi!

This MR aims to apply the TU/Package Maintainer rename to https://whatcanidofor.archlinux.org/
I also removed the specific mention of the [extra] repository as, nowadays, package maintainers have access to every repo but core*.

I'm marking it as a draft for now as we first need to update the Trusted Users Package Maintainers wiki page URL.
I'll mark it as "ready" as soon as it is done :) Ready to merge 😄