Toggle DisableLegacyMFA to true by default (#10504)

config/default.json

@@ -74,7 +74,7 @@
         "ExperimentalChannelOrganization": false,
         "EnableAPITeamDeletion": false,
         "ExperimentalEnableHardenedMode": false,
-        "DisableLegacyMFA": false,
+        "DisableLegacyMFA": true,
         "EnableEmailInvitations": false,
         "ExperimentalLdapGroupSync": false,
         "ExperimentalStrictCSRFEnforcement": false