Bump rack from 2.2.6.2 to 2.2.6.4 in /api/ruby/building-a-ci-server

Bumps [rack](https://github.com/rack/rack) from 2.2.6.2 to 2.2.6.4.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v2.2.6.2...v2.2.6.4)

---
updated-dependencies:
- dependency-name: rack
  dependency-type: indirect
...

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

Author: dependabot[bot]

api/ruby/building-a-ci-server/Gemfile.lock

@@ -10,7 +10,7 @@ GEM
       ruby2_keywords (~> 0.0.1)
     octokit (3.0.0)
       sawyer (~> 0.5.3)
-    rack (2.2.6.2)
+    rack (2.2.6.4)
     rack-protection (2.2.3)
       rack
     ruby2_keywords (0.0.5)