Bump rack from 2.2.3 to 2.2.8 in /lib/nexmo_developer

Bumps [rack](https://github.com/rack/rack) from 2.2.3 to 2.2.8.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@2.2.3...v2.2.8)

---
updated-dependencies:
- dependency-name: rack
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

lib/nexmo_developer/Gemfile.lock

@@ -380,7 +380,7 @@ GEM
       nio4r (~> 2.0)
     racc (1.6.0)
     racc (1.6.0-java)
-    rack (2.2.3)
+    rack (2.2.8)
     rack-protection (2.1.0)
       rack
     rack-proxy (0.7.0)