Update Sphinx to work with docutils-0.19.x

[ViktorHaag]

Is your feature request related to a problem? Please describe.
Docutils 0.19 has been released, and at first glance it appears to be compatible with Sphinx (cloning the Sphinx repo, running the tests clean with -e py310 and -e du19pre).

Describe the solution you'd like
Unpin Sphinx from Docutils 0.18 and include 0.19.x.

Describe alternatives you've considered
Remain pinned to 0.18.x; this may cause disruptions in projects and platforms that assume they can freely update dependencies to the latest versions, especially if they directly depend on docutils.

Additional context
One of the open issues concerning the move to docutils 0.18.x is around the change in handling with footnotes that arrived with docutils 0.18.x. One of the items in the docutils release notes for 0.19.0 states:

• Output changes:

HTML5:
Wrap groups of footnotes in an <aside> for easier styling.

The CSS rule .footnote-list { display: contents; } can be used to restore the behaviour of custom CSS > styles.

This seems like it would help address our issue #10531.

Addressed by: #10656

[AA-Turner]

You beat me to opening this issue--we test with Docutils' master weekly and I recently merged a change to test with the betas so yes good confidence on tests passing (I've just updated CI to use the released Docutils).

I'll do a visual regression test, and then update the declared support.

A