main/postmarketos-mkinitfs: upgrade to 2.1

Full changelog: https://gitlab.com/postmarketOS/postmarketos-mkinitfs/-/tags/2.1

This reverts back to using gzip for the initramfs-extra archive (cc @ollieparanoid), when we finally drop osk-sdl, then I think we can move back to using 'none' by default. Not breaking boot on some devices seemed like a higher priority than some minor speedup due to not having to decompress gzip.

The make dependency on "git" is because the makefile derives the app version from git tag info.

Fixes: postmarketos-mkinitfs#22 (closed)

Fixes: https://gitlab.com/postmarketOS/pmaports/-/issues/2009