Update concurrent-ruby requirement from < 1.3.4 to < 1.3.8 in /example

Updates the requirements on [concurrent-ruby](https://github.com/ruby-concurrency/concurrent-ruby) to permit the latest version.
- [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/v0.0.1...v1.3.7)

---
updated-dependencies:
- dependency-name: concurrent-ruby
  dependency-version: 1.3.7
  dependency-type: direct:production
...

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

example/Gemfile

@@ -7,7 +7,7 @@ ruby ">= 2.6.10"
 gem 'cocoapods', '>= 1.13', '!= 1.15.0', '!= 1.15.1'
 gem 'activesupport', '>= 6.1.7.5', '!= 7.1.0'
 gem 'xcodeproj', '< 1.26.0'
-gem 'concurrent-ruby', '< 1.3.4'
+gem 'concurrent-ruby', '< 1.3.8'
 
 # Ruby 3.4.0 has removed some libraries from the standard library.
 gem 'bigdecimal'