Stop building and deploying pre-LTS images

[juniorxsound]

What?

Right now we build and deploy all images (in manifest.json) on every push to main branch which consumes resources and is prawn to surfacing errors for legacy images that we no longer maintain. As suggested in #43 it'd be good to move to a model where we build and deploy

• Only images with LTS versions of Blender (right now 2.83 -> 2.93) as specified here https://www.blender.org/download/lts/
• Only images with LTS versions of Ubuntu (starting with 18)

[oliverpool]

If you need inspiration, I added the logic to build only one specific blender version and tag it accordingly in #47

(don't mean to spam, just for cross-referencing :)