Chore: upgrade dependencies

vueperss-next: beta.20 -> beta.21
postcss-loader: 6.1.0 -> 6.1.1

package.json

@@ -8,11 +8,11 @@
   "devDependencies": {
     "@types/bootstrap": "^5.0.16",
     "@types/jquery": "^3.5.5",
-    "@vuepress/bundler-vite": "^2.0.0-beta.20",
-    "@vuepress/plugin-back-to-top": "^2.0.0-beta.20",
-    "@vuepress/plugin-debug": "^2.0.0-beta.20",
-    "@vuepress/plugin-search": "^2.0.0-beta.20",
-    "postcss-loader": "^6.1.0",
+    "@vuepress/bundler-vite": "^2.0.0-beta.21",
+    "@vuepress/plugin-back-to-top": "^2.0.0-beta.21",
+    "@vuepress/plugin-debug": "^2.0.0-beta.21",
+    "@vuepress/plugin-search": "^2.0.0-beta.21",
+    "postcss-loader": "^6.1.1",
     "prettier": "2.3.2",
     "sass": "1.35.1",
     "sass-loader": "^12.1.0",