Skip redundant Golang upgrade jobs and add versions file

What does this merge request do and why?

Update the Golang upgrade testing workflow to skip golang-upgrade-* jobs when the current GO_VERSION matches the version in .tool-versions.

I also added .gitlab/ci/_versions.gitlab-ci.yml to define the Go version in one place, so in the future we only need to change it there.

Closes #2305 (closed)

How to set up and validate locally

Check any golang-upgrade-* jobs in https://gitlab.com/gitlab-org/gitlab-development-kit/-/pipelines/1588456665 to confirm they're skipped because the current Go version matches the target version. For example, https://gitlab.com/gitlab-org/gitlab-development-kit/-/jobs/8641475967#L1284.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.