maintainence: close release/3.4 branch

[Joibel]

3.4 is out of support and so I intend to lock down the branch.

I'd like to add a banner to read the docs (@jmeridth @tico24 if you know how to, please do this) to advise that version and the docs are un-maintainted before locking the branch.

[tico24]

I'm not aware of a brach-custom banner that can be added. I had a quick look and it looks like we can remove the branch from the docs entirely (https://docs.readthedocs.io/en/stable/versions.html#version-states) but I can't see a branch-specific banner/notification option.

[Joibel]

I'd rather leave the legacy docs around (but I'm open to arguing). Mainly I'd like to do that as this is th "one that contains all the old version info".
We can do horrific javascript things as per #13810, so maybe we'll do that.

[tico24]

oh I agree. I wasn't offering it as a real suggestion more "this seems to be their way of handling things"

[jmeridth]

Maybe adding something like https://sphinx-version-warning.readthedocs.io/?

[tico24]

Unfortunately we use mkdocs not sphinx