Update mkdocs default version

[stsewd]

Everything looking good in local testing.
The changelog doesn't mention anything breaking.

ref #7539

[Mailaender]

Seems still broken:

before: https://docs.openra.net/en/latest/release/lua/
after: https://openhv.readthedocs.io/en/latest/release/lua/

Additionally the table of contents is partly gone.

[stsewd]

@Mailaender if you are already using the latest version of mkdocs you should report those problems to mkdocs, this isn't related to rtd.

[humitos]

Won't this affect to all the builds that are not pinning mkdocs? I think what we want here is a feature flag that uses mkdocs<1.2 only for new projects.

I don't think we have a plan to update build dependencies that we install by default, but if we don't, we should so we apply it every time we need it. Like this case.

[stsewd]

Won't this affect to all the builds that are not pinning mkdocs? I think what we want here is a feature flag that uses mkdocs<1.2 only for new projects.

Yes, but this update isn't going to break the builds. Is just a minor update.

[ericholscher]

This removes python 3 support, along with a number of other large changes: https://www.mkdocs.org/about/release-notes/#other-changes-and-additions-to-version-11 -- this will definitely break things for people.

[Mailaender]

They really shouldn't have snug that into a point release.

[stsewd]

I think we can add this after #7524 is merged