Snapcraft: Update GTK+ 3 example to use core22 and potentially GTK4

Update the following how-to guide to use core22 (or core24):

https://snapcraft.io/docs/gtk3-applications

Background

Snapcraft's platform quickstart how-to guides help snap developers create a first snap with their chosen framework or development environment. Many of these guides become out of date and need to be revised. Specifically, they may need to be updated to address the following:

• update the core snap to the latest supported (such as from core18 to core22)
• an updated core will typically require changes to the associated plugin
• replace an inappropriate or no longer relevant example

Prerequisites

You will need some experience building snaps, and some knowledge of the target framework or development environment.

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