Bump @vuepress/theme-default from 2.0.0-rc.20 to 2.0.0-rc.21

Bumps [@vuepress/theme-default](https://github.com/vuejs/vuepress/tree/HEAD/packages/@vuepress/theme-default) from 2.0.0-rc.20 to 2.0.0-rc.21.
- [Release notes](https://github.com/vuejs/vuepress/releases)
- [Changelog](https://github.com/vuejs/vuepress/blob/master/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vuepress/commits/HEAD/packages/@vuepress/theme-default)

---
updated-dependencies:
- dependency-name: "@vuepress/theme-default"
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>