Leaving some maintainership positions

Hey,

From today, I won't be maintaining shared-mime-info or the xdg-specs
modules anymore. I've also relinquished my mailing-list admin position
for this list.

For shared-mime-info specifically, 16 years is probably long enough to
maintain anything.

Cheers

PS: I don't intend to remove myself as a developer from the gitlab
modules for now.