mgr/dashboard: pin xmlsec to 1.3.13

xmlsec is an inner dependency used by python3-saml. A newer release of
it broke the import.
xmlsec/python-xmlsec#314

Fixes: https://tracker.ceph.com/issues/65571
Signed-off-by: Nizamudeen A <nia@redhat.com>
(cherry picked from commit ac656f0)

src/pybind/mgr/dashboard/requirements-test.txt

@@ -3,3 +3,4 @@ pytest-instafail
 pyfakefs==4.5.0
 jsonschema~=4.0
 PyJWT~=2.0
+xmlsec==1.3.13 # Pinning because of https://github.com/xmlsec/python-xmlsec/issues/314