Update rspec-mocks requirement from ~> 3.12.6 to ~> 3.13.0

Updates the requirements on [rspec-mocks](https://github.com/rspec/rspec-mocks) to permit the latest version.
- [Release notes](https://github.com/rspec/rspec-mocks/releases)
- [Changelog](https://github.com/rspec/rspec-mocks/blob/main/Changelog.md)
- [Commits](rspec/rspec-mocks@v3.12.6...v3.13.0)

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

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

mail-notify.gemspec

@@ -35,7 +35,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "standard", "~> 1"
   spec.add_development_dependency "sqlite3", "~> 1.6.2"
   spec.add_development_dependency "webmock", "~> 3.16.0"
-  spec.add_development_dependency "rspec-mocks", "~> 3.12.6"
+  spec.add_development_dependency "rspec-mocks", "~> 3.13.0"
 
   spec.add_dependency "actionmailer", ">= 5.2.4.6"
   spec.add_dependency "activesupport", ">= 5.2.4.6"