Bump rack from 2.0.8 to 2.2.3 in /examples/streaming

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>

examples/streaming/Gemfile.lock

@@ -11,7 +11,7 @@ GEM
     eventmachine (1.2.7)
     json (2.3.0)
     mustermann (1.0.3)
-    rack (2.0.8)
+    rack (2.2.3)
     rack-protection (2.0.7)
       rack
     sinatra (2.0.7)