Update Golang to 1.21.4

What does this MR do and why?

As part of Update Golang version from 1.21.x to 1.21.4 in ... (gitlab-development-kit#1996 - closed), this MR updates the Golang version in workhorse/.tool-versions to the latest 1.21.4.

Related to gitlab-development-kit#1996 (closed)

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.