Update rspec-rails requirement from ~> 5.1 to ~> 6.1

Updates the requirements on [rspec-rails](https://github.com/rspec/rspec-rails) to permit the latest version.
- [Release notes](https://github.com/rspec/rspec-rails/releases)
- [Changelog](https://github.com/rspec/rspec-rails/blob/main/Changelog.md)
- [Commits](rspec/rspec-rails@v5.1.0...v6.1.0)

---
updated-dependencies:
- dependency-name: rspec-rails
  dependency-type: direct:development
...

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

Author: dependabot[bot]

mail-notify.gemspec

@@ -30,7 +30,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "pry", "~> 0.14.1"
   spec.add_development_dependency "rails", "7.0.8"
   spec.add_development_dependency "rake", "~> 13.0.6"
-  spec.add_development_dependency "rspec-rails", "~> 5.1"
+  spec.add_development_dependency "rspec-rails", "~> 6.1"
   spec.add_development_dependency "standard", "~> 1"
   spec.add_development_dependency "sqlite3", "~> 1.6.2"
   spec.add_development_dependency "webmock", "~> 3.16.0"