Update fast_gettext requirement from ~> 2.1 to >= 2.1, < 4.0

Updates the requirements on [fast_gettext](https://github.com/grosser/fast_gettext) to permit the latest version.
- [Changelog](https://github.com/grosser/fast_gettext/blob/master/CHANGELOG)
- [Commits](grosser/fast_gettext@v2.1.0...v2.4.0)

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

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

gettext-setup.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '>= 2.5.0'
 
-  spec.add_dependency 'fast_gettext', '~> 2.1'
+  spec.add_dependency 'fast_gettext', '>= 2.1', '< 4.0'
   spec.add_dependency 'gettext', '~> 3.4'
   spec.add_dependency 'locale'