Bump rack from 2.0.8 to 2.2.3 in /uaa/slate

Bumps [rack](https://github.com/rack/rack) from 2.0.8 to 2.2.3.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.8...2.2.3)

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

uaa/slate/Gemfile.lock

@@ -92,7 +92,7 @@ GEM
       activesupport (>= 3.1)
     parallel (1.10.0)
     public_suffix (2.0.5)
-    rack (2.0.8)
+    rack (2.2.3)
     rb-fsevent (0.9.8)
     rb-inotify (0.9.8)
       ffi (>= 0.5.0)