Bump concurrent-ruby from 1.1.7 to 1.1.8 (#15601)

Bumps [concurrent-ruby](https://github.com/ruby-concurrency/concurrent-ruby) from 1.1.7 to 1.1.8.
- [Release notes](https://github.com/ruby-concurrency/concurrent-ruby/releases)
- [Changelog](https://github.com/ruby-concurrency/concurrent-ruby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/ruby-concurrency/concurrent-ruby/compare/v1.1.7...v1.1.8

)

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Gemfile.lock

@@ -155,7 +155,7 @@ GEM
       climate_control (>= 0.0.3, < 1.0)
     coderay (1.1.3)
     color_diff (0.1)
-    concurrent-ruby (1.1.7)
+    concurrent-ruby (1.1.8)
     connection_pool (2.2.3)
     cose (1.0.0)
       cbor (~> 0.5.9)