Stop pinning Lerna to pre v3.14.0.

> In the same spirit as `apollo-server`:
> apollographql/apollo-server@585085d4d8dd7ab

Thanks to a gracious fix to Lerna by the Babel community, this issue
(originally reported in my lerna/lerna#2107) is no
longer an issue in new versions of Lerna, thanks to the fix provided in
lerna/lerna#2185.

Ref: lerna/lerna#2185
Ref: lerna/lerna#2107

cc @JakeDawkins
cc @trevor-scheer

renovate.json

@@ -13,10 +13,6 @@
       "packageNames": ["@types/node"],
       "allowedVersions": "8.x"
     },
-    {
-      "packageNames": ["lerna"],
-      "allowedVersions": "=3.13.4"
-    },
     {
       "packageNames": ["vscode-uri"],
       "allowedVersions": "=1.0.6"