WIP: [docs] [ci] always use Sphinx >=9

[jameslamb]

Based on this discussion: #6857 (comment)

• moves the Sphinx floor to 9.1.0 and updates other related package floors
• switches .. versionadded to ..version-added (see deprecation notice at https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-versionadded)

Notes for Reviewers

I've also added ..version-changed and ..version-deprecated items to the release checklist on #7129

How I tested this

Enabled this branch on readthedocs.

• build: TBD
• docs: TBD

TODO before merging

• switch .. deprecated to .. version-deprecated after [python-package] scikit-learn fit() methods: add eval_X, eval_y, deprecate eval_set #6857 is merged.

[jameslamb]

If we want to keep using breathe, we can't update to Sphinx 9 until breathe does a new release supporting it.

LibMambaUnsatisfiableError: Encountered problems while solving:
  - package breathe-4.36.0-pyhd8ed1ab_0 requires sphinx >=7.2,<9.0.0a0, but none of the providers can be installed

Could not solve for environment specs
The following packages are incompatible
├─ breathe >=4.36.0 * is installable and it requires
│  └─ sphinx >=7.2,<9.0.0a0 *, which can be installed;
└─ sphinx >=9.1.0 * is not installable because it conflicts with any installable versions previously reported.

(build link)

I'll close this for now.