Improve Ruby version upgrade template to clarify dependencies

Problem Statement

The template doesn't make it clear that related MRs should be merged together, which repos depend on others (omnibus-gitlab needs to be updated before gitlab-org/gitlab), or that we should update all repos around the same time. Some parts are outdated too, like the Ruby patch constant reference and the Remote Development GDK docker image testing step.

Impact

When repos get upgraded at different times, they end up on different Ruby versions. This can block image builds and means GDK ends up installing more than 2 Ruby versions, which makes updates slower.

Proposal

Update the template to show which repos depend on others with examples, add a checklist for updating all repos together, and remove the outdated sections.

Impacted categories

The following categories relate to this issue:

• 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.