Allow gdk update to display important messages pre and/or post update

Overview

In light of the increased effort to make gdk update update a users GDK install without breaking it, I've thought at times it would have been really handy to be able to display important messages pre and/or post gdk update to assist users in dealing with changes that may break their install. Ideally we should aim to not introduce backward incompatible changes but there have been and will continue to be (hopefully less so) occasions where this is necessary.

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.

Proposal

I'm thinking of something along the lines of:

PRE update

$ gdk update
----------------------------------------------------------------
ℹ️ PRE UPDATE HEADS UP!
----------------------------------------------------------------
Hi, this is a major upgrade to GDK so you may be asked to update
a bunch of files, please carefully review the differences :pray: 

Also, you can run `gdk doctor` at any time to give you insights 
into how to fix common issues. 🙂
----------------------------------------------------------------
-- pause --

POST update

$ gdk update
-- snip --

----------------------------------------------------------------
ℹ️ POST UPDATE HEADS UP!
----------------------------------------------------------------
Hi, you may experience an issue with X but you can overcome 
this by following the steps at Y.  If you are still experiencing 
issues, please raise an Issue at
https://gitlab.com/gitlab-org/gitlab-development-kit/issues/new

Also, you can run `gdk doctor` at any time to give you insights 
into how to fix common issues. 🙂
----------------------------------------------------------------

Thoughts?