pybind/mgr/dashboard/requirements-lint.txt: re-pin rstcheck

pybind/mgr/dashboard/requirements-lint.txt: re-pin rstcheck

As the requested Wheel for 'rstcheck==3.3.1' is not available
for Fedora 43:

Fedora 43 ships with Python 3.14 as the default interpreter.
rstcheck 3.3.1 doesn't support Python 3.14 (see supported
versions at https://pypi.org/project/rstcheck/3.3.1/)
rstcheck 6.2.0 was the first version to add Python 3.14 support
(release info: https://pypi.org/project/rstcheck/6.2.0/)

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>