Revert "chore: upgrade vitepress"

This reverts commit 7b0764c.

Author: Akryum

package.json

@@ -69,7 +69,7 @@
     "semver": "^5.5.1",
     "start-server-and-test": "^1.7.1",
     "tailwindcss": "npm:@tailwindcss/postcss7-compat",
-    "vitepress": "^0.21.6",
+    "vitepress": "^0.18.1",
     "vue-loader": "^15.9.0",
     "webpack-dev-server": "^4.0.0-beta.0"
   },