ovn: Bump ovn-nb schema version.

Commit b89d25e made the "router" DHCPv4 option optional instead of
mandatory.  This did not actually change the schema, but there's no good
way for a client of the northbound database to know if this change is
present without bumping the schema version.  This is needed for a client to
work with versions before and after this change.

Reported-at: https://bugs.launchpad.net/networking-ovn/+bug/1670666
Fixes: b89d25e ("ovn: Modify the DHCPv4 router option to optional")
Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>

ovn/ovn-nb.ovsschema

@@ -1,7 +1,7 @@
 {
     "name": "OVN_Northbound",
-    "version": "5.5.0",
-    "cksum": "2099428463 14236",
+    "version": "5.5.1",
+    "cksum": "1924970369 14236",
     "tables": {
         "NB_Global": {
             "columns": {