mgr/dashboard: replace gherkin-lint with prettier

mgr/dashboard: replace gherkin-lint with prettier

I checked a couple of gherkin linters as an alternative but all of them
are not really well maintained. There are couple of new projeects but
even they are simply maintained by a single guy so i don't think it'll
get enough contributions later on. So instead, I am moving it to
prettier. Although we miss some functional validation, it'll take care
of some basic formatting.

Fixes: https://tracker.ceph.com/issues/78528
Signed-off-by: Nizamudeen A <nia@redhat.com>