Use ubuntu-22.04 as the build OS as 20.04 will soon be EOL.

Update to Python 3.10 as 3.8 will soon be EOL. 3.10 is the Python
version distributed by Ubuntu 22.04 and also the version used by
Read the Doc's own documentation.

Update Gitlab CI docs build job to use the same Python version.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>