chore(deps-dev): bump the vuepress group with 2 updates

Bumps the vuepress group with 2 updates: [@vuepress/plugin-shiki](https://github.com/vuepress/vuepress-next) and [@vuepress/theme-default](https://github.com/vuejs/vuepress/tree/HEAD/packages/@vuepress/theme-default).


Updates `@vuepress/plugin-shiki` from 2.0.0-rc.120 to 2.0.0-rc.121
- [Release notes](https://github.com/vuepress/vuepress-next/releases)
- [Changelog](https://github.com/vuepress/core/blob/main/CHANGELOG.md)
- [Commits](https://github.com/vuepress/vuepress-next/commits)

Updates `@vuepress/theme-default` from 2.0.0-rc.120 to 2.0.0-rc.121
- [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/plugin-shiki"
  dependency-version: 2.0.0-rc.121
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: vuepress
- dependency-name: "@vuepress/theme-default"
  dependency-version: 2.0.0-rc.121
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: vuepress
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]