Update Golang to 1.21.4

What does this MR do?

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)

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines