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
ceph#62239, but that is failing with some
other errors.

Fixes: https://tracker.ceph.com/issues/70411
Signed-off-by: Nizamudeen A <[email protected]>

Author: nizamial09

src/pybind/mgr/dashboard/requirements.txt

@@ -14,3 +14,4 @@ jsonpatch
 grpcio==1.46.5
 grpcio-tools==1.46.5
 jmespath
+lxml==4.8.0  # to fix https://github.com/xmlsec/python-xmlsec/issues/320