Bump rack from 3.0.11 to 3.1.21 (#23)

Bumps [rack](https://github.com/rack/rack) from 3.0.11 to 3.1.21.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](https://github.com/rack/rack/commits/v3.1.21)

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

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Author: dependabot[bot]

Gemfile.lock

@@ -86,7 +86,7 @@ GEM
     psych (5.1.2)
       stringio
     racc (1.8.1)
-    rack (3.0.11)
+    rack (3.1.21)
     rack-session (2.0.0)
       rack (>= 3.0.0)
     rack-test (2.1.0)