Update public_suffix requirement from >= 3.0, < 5.0 to >= 3.0, < 6.0

Updates the requirements on [public_suffix](https://github.com/weppos/publicsuffix-ruby) to permit the latest version.
- [Release notes](https://github.com/weppos/publicsuffix-ruby/releases)
- [Changelog](https://github.com/weppos/publicsuffix-ruby/blob/main/CHANGELOG.md)
- [Commits](weppos/publicsuffix-ruby@v3.0.0...v3.1.1)

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

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

Author: dependabot[bot]

github-pages-health-check.gemspec

@@ -19,6 +19,6 @@ Gem::Specification.new do |s|
   s.add_dependency("addressable", "~> 2.3")
   s.add_dependency("dnsruby", "~> 1.60")
   s.add_dependency("octokit", "~> 4.0")
-  s.add_dependency("public_suffix", ">= 3.0", "< 5.0")
+  s.add_dependency("public_suffix", ">= 3.0", "< 6.0")
   s.add_dependency("typhoeus", "~> 1.3")
 end