We need a short term bridge of support for ubuntu 20.04 while working on Cluster API migration

[Michael-Sinz]

Describe the request We need to build clusters with Ubuntu 20.04 as 18.04 is hitting end of support on 4/2023

Explain why AKS Engine needs it Just needs it long enough such that we can keep our services compliant until we finish the move to Cluster API

Describe the solution you'd like Full support for 20.04 - it could be done as a replacement for 18.04 if needed

Describe alternatives you've considered Cluster API is in the works but there are a lot of things yet to be covered and we need something we can roll out to remain compliant until that is ready to handle our technologies.

Additional context