Bump Ruby version in source installation docs to 2.7.6
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
- Install Ruby 2.7.6 according to source install docs
- 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.
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I have evaluated the MR acceptance checklist for this MR.