chore: bump puma in /integration/examples/ruby/backend (#9244)

Bumps [puma](https://github.com/puma/puma) from 5.6.7 to 5.6.8.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v5.6.7...v5.6.8)

---
updated-dependencies:
- dependency-name: puma
  dependency-type: direct:production
...

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integration/examples/ruby/backend/Gemfile.lock

@@ -1,8 +1,8 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    nio4r (2.5.9)
-    puma (5.6.7)
+    nio4r (2.7.0)
+    puma (5.6.8)
       nio4r (~> 2.0)
     rack (2.2.6.4)
     rack-unreloader (1.7.0)