Update Mockttp for trailer support & HTTP/2 improvements

Notably this now reports :status in HTTP/2 header events, properly
forwards trailers where possible, and generally brings HTTP/2 to a state
of full maturity where I think it's time to now enable it by default.

Author: pimterry

package-lock.json

@@ -76,7 +76,7 @@
     "mime-types": "^2.1.27",
     "mobx": "^6.3.5",
     "mockrtc": "^0.3.1",
-    "mockttp": "^3.10.1",
+    "mockttp": "^3.11.0",
     "node-fetch": "^2.6.1",
     "node-forge": "^1.3.0",
     "node-gsettings-wrapper": "^0.5.0",