fix(deps): update dependency @vuepress/plugin-feed to v2.0.0-rc.21

Author: renovate[bot]

package.json

@@ -19,7 +19,7 @@
     "@vuepress/bundler-vite": "2.0.0-rc.8",
     "@vuepress/helper": "2.0.0-rc.15",
     "@vuepress/plugin-copy-code": "2.0.0-rc.15",
-    "@vuepress/plugin-feed": "2.0.0-rc.15",
+    "@vuepress/plugin-feed": "2.0.0-rc.21",
     "@vueuse/core": "10.8.0",
     "artplayer": "5.1.1",
     "axios": "1.6.7",