Inventory of core libraries maintenance status

Following https://github.com/haskell/mtl/issues/158#issuecomment-3197794496 I want to make an inventory of maintenance status.

This is mainly for my own notes, but other CLC members may chime in if they want to do so.

I want to go through the list of core libraries and:

* check if there was activity in the last 6 months (either on hackage or the source repository)
* if there was no activity, contact the maintainers (either via github/gitlab/etc. issue or email)