Update tinymce-rails requirement from ~> 5.0 to >= 5, < 8

Updates the requirements on [tinymce-rails](https://github.com/spohlenz/tinymce-rails) to permit the latest version.
- [Release notes](https://github.com/spohlenz/tinymce-rails/releases)
- [Commits](spohlenz/tinymce-rails@v5.0.0...v7.0.0)

---
updated-dependencies:
- dependency-name: tinymce-rails
  dependency-type: direct:production
...

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

spree_backend.gemspec

@@ -43,5 +43,5 @@ Gem::Specification.new do |s|
   s.add_dependency 'sass-rails',       '>= 5'
   s.add_dependency 'select2-rails',    '~> 4.0.6'
   s.add_dependency 'sprockets',        '~> 4.0'
-  s.add_dependency 'tinymce-rails',    '~> 5.0'
+  s.add_dependency 'tinymce-rails',    '>= 5', '< 8'
 end