Corrected minimum Go version required (15.2 requires Go 1.17+)

What does this MR do and why?

This MR corrects a mistake in the Software Requirements table - it incorrectly says Go 1.16 is the minimum version, but actually Go 1.17 or later is required (GitLab 15.2 fails to install with Go 1.16).

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Install Go 1.16 (for example, https://go.dev/dl/go1.16.7.linux-amd64.tar.gz)
2. Change into the Gitaly folder (e.g. cd /home/git/gitaly if using a source build)
3. Attempt to compile Gitaly (sudo -u git -H make all)
4. Compilation will fail due to the following error:

   //go:build comment without // +build comment

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.