Update puppet_forge requirement from >= 4.1, < 6 to >= 4.1, < 7

Updates the requirements on [puppet_forge](https://github.com/puppetlabs/forge-ruby) to permit the latest version.
- [Release notes](https://github.com/puppetlabs/forge-ruby/releases)
- [Changelog](https://github.com/puppetlabs/forge-ruby/blob/main/CHANGELOG.md)
- [Commits](puppetlabs/forge-ruby@v4.1.0...v5.0.4)

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

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

r10k.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'log4r',     '1.1.10'
   s.add_dependency 'multi_json', '~> 1.10'
 
-  s.add_dependency 'puppet_forge', '>= 4.1', '< 6'
+  s.add_dependency 'puppet_forge', '>= 4.1', '< 7'
 
   s.add_dependency 'gettext-setup', '>=0.24', '<2.0'