mgr/dashboard: pin lxml to fix run-dashboard-tox-make-check failure

mgr/dashboard: pin lxml to fix run-dashboard-tox-make-check failure

xmlsec had an upgrade yesterday night and python3-saml might need to
adapt its library accordingly I suppose. Testing a fix by pinning lxml

Another approach is being tried out separately
https://github.com/ceph/ceph/pull/62239, but that is failing with some
other errors.

Fixes: https://tracker.ceph.com/issues/70411
Signed-off-by: Nizamudeen A <nia@redhat.com>
(cherry picked from commit 1f84505f1232dd8445df1a2a819fa000062d3934)

 Conflicts:
	src/pybind/mgr/dashboard/requirements.txt
 - didn't add the dependency that are present in main and only added the
   lxml pinning