[Security] Bump rack from 2.0.7 to 2.0.8

Bumps [rack](https://github.com/rack/rack) from 2.0.7 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@2.0.7...2.0.8)

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

Author: dependabot-preview[bot]

Gemfile.lock

@@ -2,7 +2,7 @@ GEM
   remote: http://rubygems.org/
   specs:
     mustermann (1.0.3)
-    rack (2.0.7)
+    rack (2.0.8)
     rack-protection (2.0.7)
       rack
     redis (4.1.3)