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 .tool-versions
to the latest 1.21.4.
Related to gitlab-development-kit#1996 (closed)
How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.