Bump rack from 3.1.7 to 3.1.21

Bumps [rack](https://github.com/rack/rack) from 3.1.7 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@v3.1.7...v3.1.21)

---
updated-dependencies:
- dependency-name: rack
  dependency-version: 3.1.21
  dependency-type: direct:development
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -169,7 +169,7 @@ GEM
       forwardable-extended (~> 2.6)
     public_suffix (6.0.1)
     racc (1.8.1)
-    rack (3.1.7)
+    rack (3.1.21)
     rack-test (2.1.0)
       rack (>= 1.3)
     rackup (2.1.0)