[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](https://github.com/rack/rack/compare/2.0.7...2.0.8)

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

Gemfile.lock

@@ -9,7 +9,7 @@ GEM
     minitest (5.12.2)
     mustermann (1.0.3)
     open4 (1.3.4)
-    rack (2.0.7)
+    rack (2.0.8)
     rack-protection (2.0.7)
       rack
     rake (12.3.3)