allow using octokit 6.x

[pravi]

Pleas relax the requirement for octokit to allow using 6.x versions or even upto 8.x like github-pages-health-check

For example https://github.com/github/pages-health-check/blob/120cc09f776f42e9ac4e9d87e66a042819b8aa32/github-pages-health-check.gemspec already has a very relaxed requirement for octokit

5.0 just dropped support for ruby versions without support https://github.com/octokit/octokit.rb/releases/tag/v5.0.0
6.0 is probably fine (removed functionalities that were not operational) https://github.com/octokit/octokit.rb/releases/tag/v6.0.0