Bump @vuepress/plugin-back-to-top from 1.9.9 to 1.9.10

Bumps [@vuepress/plugin-back-to-top](https://github.com/vuejs/vuepress/tree/HEAD/packages/@vuepress/plugin-back-to-top) from 1.9.9 to 1.9.10.
- [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/v1.9.10/packages/@vuepress/plugin-back-to-top)

---
updated-dependencies:
- dependency-name: "@vuepress/plugin-back-to-top"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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