Change default branch to something other than MOODLE_311_STABLE

[lucaboesch]

Don't know what others think about but seeing MOODLE_311_STABLE as default branch when accessing this repository always gives me very nostalgic vibes.
(Moodle 3.11 was released in 2021…)

Please consider changing to somewhat more modern, e.g. MOODLE_404_DEV.

[lucaboesch]

Now (2025-01-28) that you've done some work it is more and more incomprehensible that you did not change the default branch to something more actual.