Update stylelint from v9.10.1 to v13.6.1 (#9001)

The changes made between v9.10.1 and v13.6.1 don't appear to be
relevant to us, aside from bug fixes that we'd benefit from.

`gulp-stylelint` also needed to be updated, as it's in-step with
`stylelint`. It went from v7 to v13.0.0. The changes aren't notable
here for us either.

package.json

@@ -235,7 +235,7 @@
     "gulp-rtlcss": "^1.4.0",
     "gulp-sass": "^4.0.0",
     "gulp-sourcemaps": "^2.6.0",
-    "gulp-stylelint": "^7.0.0",
+    "gulp-stylelint": "^13.0.0",
     "gulp-terser-js": "^5.0.0",
     "gulp-watch": "^5.0.1",
     "gulp-zip": "^4.0.0",
@@ -269,7 +269,7 @@
     "source-map-explorer": "^2.0.1",
     "string.prototype.matchall": "^4.0.2",
     "style-loader": "^0.21.0",
-    "stylelint": "^9.10.1",
+    "stylelint": "^13.6.1",
     "through2": "^2.0.3",
     "ttest": "^2.1.1",
     "vinyl-buffer": "^1.0.1",