Remove osk-sdl

• Make unl0kr the default in postmarketOS edge (!3279 (merged))
• Publish edge blog post that explains how to migrate to unl0kr and why osk-sdl is deprecated (https://gitlab.com/postmarketOS/postmarketos.org/-/merge_requests/231)
• Add very visible warning in postmarketos-mkinitfs when osk-sdl is still used, pointing to the edge blog post (I think that's the best way to make people aware who still have osk-sdl from existing installs) (https://gitlab.com/postmarketOS/postmarketos-mkinitfs/-/merge_requests/43)
• Add note to osk-sdl.git repository that this is deprecated in favor of unl0kr, point at the blog post.
• After v23.12 is out: Let unl0kr provide osk-sdl and remove osk-sdl from pmaports.git
  • publish edge blog post when it's done
• Alpine's aports: finally remove deprecated directfb, and sdl2's dependency on it
• Remove osk-sdl support in postmarketos-mkinitfs (https://gitlab.com/postmarketOS/postmarketos-mkinitfs/-/merge_requests/42)

CC: @cherrypicker, @craftyguy