Upgrade sphinx version

[DelazJ]

With Sphinx 4.x being more and more deprecated, we need to either pin our dependencies or upgrade (see https://github.com/qgis/QGIS-Website/pull/1135#issuecomment-1890916531). This PR proposes to upgrade to a more recent Sphinx version (pinned at 7.2.6) with a unpinned sphinx-rtd-theme.
While upgrading, I faced some other issues with setting the locale (see sphinx-doc/sphinx#11739) and this PR tries to fix them (at least for the github actions). Basically from what I could find, you need to locally set the LC_ALL locale env to C.UTF-8 but I'm not comfortable with this kind of things and:

• it looks to me like a workaround
• on a local build, people have to set that variable: what is its scope actually? Would that "break" anything on their system? Is it possible to set that env only in the venv they are running? <-- I'm asking this because we need to instruct people that new requirement.
• I can't find how to do the same on macOS and I didn't test Windows. So if you know a better way to solve that locale issue on all platforms, ...

[DelazJ]

Let's do the changes in the website requirements file