Update rspec-rails requirement from ~> 5 to ~> 6

Updates the requirements on [rspec-rails](https://github.com/rspec/rspec-rails) to permit the latest version.
- [Changelog](https://github.com/rspec/rspec-rails/blob/main/Changelog.md)
- [Commits](rspec/rspec-rails@v5.0.0...v6.1.3)

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

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

Author: dependabot[bot]

govuk_admin_template.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |gem|
 
   gem.add_development_dependency "capybara", "~> 3"
   gem.add_development_dependency "climate_control", "~> 1"
-  gem.add_development_dependency "rspec-rails", "~> 5"
+  gem.add_development_dependency "rspec-rails", "~> 6"
   gem.add_development_dependency "rubocop-govuk", "5.0.2"
   gem.add_development_dependency "sassc-rails", "~> 2"
 end