Update bootstrap requirement from ~> 4.0 to >= 4, < 6

Updates the requirements on [bootstrap](https://github.com/twbs/bootstrap-rubygem) to permit the latest version.
- [Release notes](https://github.com/twbs/bootstrap-rubygem/releases)
- [Changelog](https://github.com/twbs/bootstrap-rubygem/blob/main/CHANGELOG.md)
- [Commits](twbs/bootstrap-rubygem@v4.0.0...v5.3.3)

---
updated-dependencies:
- dependency-name: bootstrap
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

spree_backend.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'spree', ">= 4.7.0"
 
   s.add_dependency 'babel-transpiler', '~> 0.7'
-  s.add_dependency 'bootstrap',        '~> 4.0'
+  s.add_dependency 'bootstrap',        '>= 4', '< 6'
   s.add_dependency 'flag-icons-rails', '~> 3.4'
   s.add_dependency 'flatpickr', '~> 4.6'
   s.add_dependency 'glyphicons', '~> 1.0'