Use go 1.21.7 as default and drop support for 1.19

What does this MR do?

Pipeline in Update module github.com/prometheus/client_gola... (!964 - merged) • GitLab Dependency Bot failed because prometheus library now requires at least go 1.20.

We've upgraded to Go 1.20+ more than 6 months ago: https://gitlab.com/gitlab-org/build/CNG/-/blame/e47ab6334d2853e395b01072c84bef23f2c5d4b0/ci_files/variables.yml?page=1#L29, so it's safe to drop support for Go 1.19

This MR:

• makes 1.21 a default for tests
• drops support for 1.19
• updates .tool-versions to match the current version in gdk which is set from runner: https://gitlab.com/gitlab-org/gitlab-runner/-/raw/main/.tool-versions

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