Bump @docusaurus/preset-classic from 2.0.0-beta.20 to 2.0.0-beta.21 (#1351)

Bumps [@docusaurus/preset-classic](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus-preset-classic) from 2.0.0-beta.20 to 2.0.0-beta.21.
- [Release notes](https://github.com/facebook/docusaurus/releases)
- [Changelog](https://github.com/facebook/docusaurus/blob/main/CHANGELOG.md)
- [Commits](https://github.com/facebook/docusaurus/commits/v2.0.0-beta.21/packages/docusaurus-preset-classic)

---
updated-dependencies:
- dependency-name: "@docusaurus/preset-classic"
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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package.json

@@ -13,7 +13,7 @@
   },
   "dependencies": {
     "@docusaurus/core": "^2.0.0-beta.21",
-    "@docusaurus/preset-classic": "^2.0.0-beta.20",
+    "@docusaurus/preset-classic": "^2.0.0-beta.21",
     "@easyops-cn/docusaurus-search-local": "^0.25.0",
     "@mdx-js/react": "^1.6.21",
     "by-node-env": "^2.0.1",