Bump Ruby version in source installation docs to 2.7.6

Merged Jörg Behrmann requested to merge behrmann/gitlab:ruby276 into master

What does this MR do and why?

The source install and upgrade documentation still refers to Ruby 2.7.5, but the current upstream release is 2.7.6, which fixes "CVE-2022-28739: Buffer overrun in String-to-Float conversion".

How to set up and validate locally

  1. Install Ruby 2.7.6 according to source install docs
  2. Restart GitLab## MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.