Update dependency purgecss to v8 (#263)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "metalsmith-css-unused",
-  "version": "2.0.3",
+  "version": "2.0.4",
   "description": "A Metalsmith plugin to remove unused CSS rules.",
   "keywords": [
     "metalsmith",
@@ -49,7 +49,7 @@
   },
   "dependencies": {
     "deepmerge": "^4.3.1",
-    "purgecss": "^7.0.2"
+    "purgecss": "^8.0.0"
   },
   "peerDependencies": {
     "metalsmith": "^2.5.0"