[MM-14081] Disable checkMFA Endpoint by default and add tests for MFA login (#10356)

config/default.json

@@ -73,6 +73,7 @@
         "ExperimentalChannelOrganization": false,
         "EnableAPITeamDeletion": false,
         "ExperimentalEnableHardenedMode": false,
+        "DisableLegacyMFA": true,
         "EnableEmailInvitations": false,
         "ExperimentalLdapGroupSync": false,
         "ExperimentalStrictCSRFEnforcement": false