Update node-fetch dependency & lockfile version (#3670)

Modern versions of node and npm should be compatible with this format.

Supersedes https://github.com/matrix-org/matrix-doc/pull/3668

package.json

@@ -18,10 +18,9 @@
     "url": "https://github.com/matrix-org/matrix-doc"
   },
   "homepage": "https://github.com/matrix-org/matrix-doc#readme",
-  "dependencies": {},
   "devDependencies": {
     "autoprefixer": "^9.8.6",
-    "node-fetch": "^2.6.1",
+    "node-fetch": "^2.6.7",
     "postcss-cli": "^7.1.2"
   }
 }