Add old NodeJS 20.12 as secondary option to .tool-versions

What does this merge request do and why?

Add old NodeJS 20.12 as secondary option to .tool-versions

NodeJS 22.12 is still the default and 20.12 is now installed as well.

This accounts for the mismatch between GDK's tool-versions and monolith's tool-versions.

Refs

• #2800 (closed)
• !4977 (merged)

How to set up and validate locally

1. mise list node:

$ mise list node
Tool  Version  Source                             Requested
node  18.18.0
node  20.12.2  ~/devel/gitlab/gdk/.tool-versions  20.12.2
node  20.15.0
node  20.17.0
node  20.18.0
node  22.11.0
node  22.12.0  ~/devel/gitlab/gdk/.tool-versions  22.12.0
node  22.13.1
node  22.14.0
node  22.16.0

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 MR references an issue describing the change.
• 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.