Freeze toolchain images and introduce "next" build

What does this MR do?

Stop rebuilding base and toolchain images for High Sierra, Mojave, Catalina and Big Sur.

Adds a new next category, which runs on the latest OS and toolchain versions, and is rebuilt on each pipeline (no change detection)

Why was this MR needed?

Implementing image freezing from !84 (closed)

What's the best way to test this MR?

What are the relevant issue numbers?