build(deps): bump rack from 3.1.20 to 3.1.21 in /docs/v3 (#4994)

Bumps [rack](https://github.com/rack/rack) from 3.1.20 to 3.1.21.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v3.1.20...v3.1.21)

---
updated-dependencies:
- dependency-name: rack
  dependency-version: 3.1.21
  dependency-type: indirect
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/v3/Gemfile.lock

@@ -129,7 +129,7 @@ GEM
     prism (1.9.0)
     public_suffix (7.0.2)
     racc (1.8.1)
-    rack (3.1.20)
+    rack (3.1.21)
     rack-livereload (0.6.1)
       rack (>= 3.0, < 3.2)
     rackup (2.3.1)