Unpin rstcheck

[samccann]

We've had rstcheck pinned since ansible/ansible#78478 (comment).

That was over a year ago. Since there doesn't seem to be movement in what we think is the underlying cause upstream, let's see if we can work around the problem with our snippets and get rid of that pinning.

Forum discussion at https://forum.ansible.com/t/figuring-out-how-to-upgrade-rstcheck-to-6-for-docs-builds/7428

[samccann]

rstcheck migration guide - https://rstcheck.readthedocs.io/en/latest/migrations/