Replace broken consultancy link in README

What does this MR do and why?

This replaces the broken Consultancy (https://about.gitlab.com/consultancy/) link in the README and replaces it with a link to the Professional Services page on about.gitlab.com, which also includes information about GitLab’s consultancy services.

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.