Bump @webref/css in the development-dependencies group

Bumps the development-dependencies group with 1 update: [@webref/css](https://github.com/w3c/webref).


Updates `@webref/css` from 6.22.0 to 6.23.4
- [Commits](https://github.com/w3c/webref/compare/@webref/[email protected]...@webref/[email protected])

---
updated-dependencies:
- dependency-name: "@webref/css"
  dependency-version: 6.23.4
  dependency-type: direct:development
  update-type: version-update:semver-minor
  dependency-group: development-dependencies
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package-lock.json

@@ -39,7 +39,7 @@
 		"postcss": "^8.4"
 	},
 	"devDependencies": {
-		"@webref/css": "6.22.0",
+		"@webref/css": "6.23.4",
 		"css-tree": "^3.1.0"
 	},
 	"scripts": {

packages/css-syntax-patches-for-csstree/package.json

@@ -123,7 +123,7 @@
 	"devDependencies": {
 		"@csstools/postcss-bundler": "*",
 		"@csstools/postcss-tape": "*",
-		"@webref/css": "6.22.0"
+		"@webref/css": "6.23.4"
 	},
 	"scripts": {
 		"prebuild": "node ./scripts/generate-plugins-data.mjs && eslint --fix ./src/plugins/*.mjs",