Smarter cleanup of Tier2 images

Looks like it's very common practice that cleanup Tier2 images when run out of storage.

But our provisioning scripts install packages without fixed versions normally. Then re-generated Tier2 images are not same as previous ones. There is risk to get regression.

And also it takes time to re-generate.

Could we have smarter strategy for Tier2 images cleanup?

The recent situation is like:

• 5.12.4 and 5.13.0 were released
• qt5 5.12.4->5.12 and qt5 5.13.0->5.13 merges are done

Then we can remove the Tier2 images related with 5.12.4 and 5.13.0, and old 5.12, 5.13 baseline. But keep the ones for new 5.12 and 5.13.

But perhaps there are more factors, for example, there are still some QtAutomotive or other products based on 5.12.4 and 5.13.0 baseline. Anyway, the old 5.12 and 5.13 baseline are not needed.