WIP Replace Raspbian-based images with Ubuntu Core based images - closes #2801

[benfrancis]

Closes #3139

This PR would entirely remove Raspbian-based builds from the release workflow and replace them with Ubuntu Core model assertions for AMD64 (e.g. Intel NUC) and ARM64 (e.g. Raspberry Pi), with a view to using Ubuntu Core as the basis for future OS images, as discussed in #2801.

TODO:

• Test ARM-based images once an ARM-based snap is successfully released on the edge channel
• Consider adding 32bit images on ARM
• Create a new build script for Ubuntu Core-based builds to be run from the GitHub release workflow which basically calls ubuntu-image snap and uploads the resulting compressed images to the GitHub release
• Double check nobody wants to continue to maintain the Raspbian images before removing the build script completely (note that builds will be broken after the upgrade to Node.js 20 since Raspbian Buster doesn't support that version of Node.js)