ci: remove hacks for docs building

[flying-sheep]

Inspired by @eroell: https://scverse.zulipchat.com/#narrow/channel/316218-repo-management/topic/cookiecutter.20template.20release/near/566723956

On Python ≤3.13, sphinx-autodoc-typehints uses :data:`typing.Union` which only works when linking to https://docs.python.org/3.13 (or lower).

There are two ways to remove the hack, both of which used in here:

1. since RTD supports Python 3.14 now, build docs with 3.14
2. set always_use_bars_union = True (also done, so the docs hatch env can use any Python version)

[github-actions]

A PR has been generated to the instance repo:
scverse/cookiecutter-scverse-instance#250

You can check out the PR to preview your changes
in an instance of the cookiecutter template. The PR will be kept in sync with
this PR automatically.

[Zethson]

Thank you!