Update documentation to use Teleport for generating SAAS service ping

What does this MR do and why?

We're moving to Teleport for generating service ping for GitLab.com, hence need to update the documentation.

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 #341912 (closed)