chg: doc: Revert "Add Sphinx 9.1.0"

This reverts commit a31a4c2d2d818030a2f2513d18361ef1e2be72b7.

ReadTheDocs will not work with this version:

    python -m pip install -r https://gitlab.isc.org/isc-projects/bind9/-/raw/main/doc/arm/requirements.txt
    ERROR: Ignored the following yanked versions: 8.3.0
    ERROR: Ignored the following versions that require a different python version: 9.1.0 Requires-Python >=3.12; 9.1.0rc1 Requires-Python >=3.12; 9.1.0rc2 Requires-Python >=3.12
    ERROR: Could not find a version that satisfies the requirement Sphinx==9.1.0 (from versions: 0.1.61611, ..., 9.0.4)
    ERROR: No matching distribution found for Sphinx==9.1.0

Merge branch 'mnowak/revert-sphinx-9.1.0' into 'main'

See merge request isc-projects/bind9!11793