Update Rack to v2.2.3

Rack v2.1.4 already has the CVEs fixed in v2.2.3, but we might as well
upgrade to the latest version to keep up with the latest changes. Plus,
we are already shipping Rack v2.2.3 with gitlab-exporter and Gitaly, so
eliminates some gem duplication.

List of changes:
https://github.com/rack/rack/blob/master/CHANGELOG.md