chore: Use a more specific engines.npm value.

Follows up on 068a636 but using precisely what is in
Renovate's documentation since the previous attempt (6.x) didn't work.

Ref: https://docs.renovatebot.com/node/#configuring-which-version-of-npm-renovate-uses

package.json

@@ -22,7 +22,7 @@
   },
   "engines": {
     "node": ">=6 <15",
-    "npm": "6.x"
+    "npm": "^6.14.11"
   },
   "dependencies": {
     "@apollographql/apollo-tools": "0.4.9",