Update rspec-rails versions

These versions were very old. Version 5.x is for Rails 5.2 and
6.x. Recently we removed 7.0 from our CI tests so these are much older
  than we are testing. We added Rails 8 to the CI tests which means we
should be using rspec-rails 8.x.

Author: AlistairNorman

solidus_dev_support.gemspec

@@ -39,7 +39,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency "github_changelog_generator", "~> 1.15"
   spec.add_dependency "puma", ">= 4.3", "< 7.0"
   spec.add_dependency "rspec_junit_formatter"
-  spec.add_dependency "rspec-rails", ">= 5.0", "< 7.0"
+  spec.add_dependency "rspec-rails", ">= 6.0", "< 9.0"
   spec.add_dependency "selenium-webdriver", "~> 4.11"
   spec.add_dependency "simplecov-cobertura", "~> 2.1"
   spec.add_dependency "solidus_core", [">= 2.0", "< 5"]