Update rspec-rails versions

These versions were very old. Version 5.x is for Rails 5.2 and
6.x. 7.0 was removed 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"]