doc: Update the minimum Git version to 2.37.0

What does this MR do and why?

From 15.6 onwards the minimum Git version (used by Gitaly) will be v2.37.0 and onward. So update the docs to reflect this.

Screenshots or screen recordings

Documentation change!

How to set up and validate locally

Not required!

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Related to #4192