Fix mismatch between docutils version in CI and ReadTheDocs

Fix mismatch between docutils version in CI and ReadTheDocs

Currently our CI images we use to build docs (which subsequently get
into release tarballs) are using docutils 0.17.1, which is latest version
which fulfills Sphinx 4.5.0 requirement for docutils < 0.18.

The old requirement for docutils < 0.17 was causing discrepancy between
the way we build release artifacts and the docs on ReadTheDocs.org which
uses doc/arm/requirements.txt from our repo.

Remove the limit for RDT with hope that it will pull latest permissible
version of docutils.

For the long history of messing with docutils version read also
isc-projects/images@d4435b97be70c2a3a6f8b570ef0dcb616c3dc4ae
isc-projects/bind9@6a2daddf5b95375668945801f636c8335e300fb5