Update notifications-ruby-client requirement from ~> 5.1 to >= 5.1, <…

… 7.0

Updates the requirements on [notifications-ruby-client](https://github.com/alphagov/notifications-ruby-client) to permit the latest version.
- [Changelog](https://github.com/alphagov/notifications-ruby-client/blob/main/CHANGELOG.md)
- [Commits](alphagov/notifications-ruby-client@5.1.1...6.0.0)

---
updated-dependencies:
- dependency-name: notifications-ruby-client
  dependency-type: direct:production
...

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

mail-notify.gemspec

@@ -41,6 +41,6 @@ Gem::Specification.new do |spec|
   spec.add_dependency "activesupport", ">= 5.2.4.6"
   spec.add_dependency "actionpack", ">= 5.2.7.1"
   spec.add_dependency "actionview", ">= 5.2.7.1"
-  spec.add_dependency "notifications-ruby-client", "~> 5.1"
+  spec.add_dependency "notifications-ruby-client", ">= 5.1", "< 7.0"
   spec.add_dependency "rack", ">= 2.1.4.1"
 end