Update Mockttp for tiny shutdown log noise bugfix

Without this, if the UI was unlinked from the server (e.g. by a server
restart), the resulting requests triggered a bunch of error noise on
the server end. Messy & annoying.

Author: pimterry

package-lock.json

@@ -54,7 +54,7 @@
     "lodash": "^4.17.21",
     "lookpath": "^1.2.1",
     "mime-types": "^2.1.27",
-    "mockttp": "^2.1.0",
+    "mockttp": "^2.1.1",
     "node-fetch": "^2.6.1",
     "node-forge": "^0.10.0",
     "node-gsettings-wrapper": "^0.5.0",