RTD deprecated ubuntu 20.04

[braingram]

asdf/.readthedocs.yaml

Line 6 in 64eb0eb

os: "ubuntu-20.04"

see:
https://about.readthedocs.com/blog/2026/03/ubuntu-20-04-deprecated/

we can likely just bump this to 24.04.

Impacts:
asdf
asdf-coordinates-schemas
asdf-standard
asdf-transform-schemas
asdf-wcs-schemas

possibly others (the above list was copied from an email sent to me by RTD).

[sydduckworth]

Closed by

• Update RTD build OS from Ubuntu 20.04 to 24.04 #2013
• Update RTD build OS from Ubuntu 20.04 to 24.04 asdf-standard#503
• Update RTD build OS from Ubuntu 20.04 to 24.04 asdf-wcs-schemas#84
• Update RTD build OS from Ubuntu 20.04 to 24.04 asdf-coordinates-schemas#81
• Update RTD build OS from Ubuntu 20.04 to 24.04 asdf-transform-schemas#134