[Security] Bump rack from 2.0.6 to 2.0.8

Bumps [rack](https://github.com/rack/rack) from 2.0.6 to 2.0.8. **This update includes a security fix.**
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.6...2.0.8)

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

Author: dependabot-preview[bot]

Gemfile.lock

@@ -271,7 +271,7 @@ GEM
       coderay (~> 1.1.0)
       method_source (~> 0.9.0)
     public_suffix (3.0.3)
-    rack (2.0.6)
+    rack (2.0.8)
     rainbow (3.0.0)
     rake (12.3.1)
     rb-fsevent (0.10.3)