chore: Upgrade stylelint

For a while, we were forced to use a fork of stylelint with a
bugfix necessary for our test bots to run the tool.
That behavior has now been merged into normal stylelint, so we
can stop using that fork.

Change-Id: I51196f8dab73ac1f814137d2f191a08c231fd369

package.json

@@ -54,7 +54,7 @@
     "pwacompat": "^2.0.10",
     "rimraf": "^2.6.3",
     "sprintf-js": "^1.1.2",
-    "stylelint": "github:theodab/stylelint#941a097114756bf2bd8a4d4dff1f659bcc520eb4",
+    "stylelint": "^13.8.0",
     "stylelint-config-standard": "^20.0.0",
     "tippy.js": "^4.3.1",
     "ua-parser-js": "github:joeyparrish/ua-parser-js#develop",