Update shoulda-matchers requirement from ~> 4.4 to ~> 6.4

Updates the requirements on [shoulda-matchers](https://github.com/thoughtbot/shoulda-matchers) to permit the latest version.
- [Release notes](https://github.com/thoughtbot/shoulda-matchers/releases)
- [Changelog](https://github.com/thoughtbot/shoulda-matchers/blob/main/CHANGELOG.md)
- [Commits](thoughtbot/shoulda-matchers@v4.4.0...v6.4.0)

---
updated-dependencies:
- dependency-name: shoulda-matchers
  dependency-type: direct:development
...

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

Author: dependabot[bot]

solidus_subscriptions.gemspec

@@ -37,7 +37,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'state_machines'
 
   spec.add_development_dependency 'rspec-activemodel-mocks'
-  spec.add_development_dependency 'shoulda-matchers', '~> 4.4'
+  spec.add_development_dependency 'shoulda-matchers', '~> 6.4'
   spec.add_development_dependency 'solidus_dev_support', '~> 2.0'
   spec.add_development_dependency 'timecop'
   spec.add_development_dependency 'vcr'