[docker-in-docker] - Remove default value for "mobyBuildxVersion"

[samruddhikhandale]

From #838 and #837

Remove default value for "mobyBuildxVersion" once Microsoft fixes the issue

@samruddhikhandale Let's see what GHA has to say about the latest commit. It's a little ugly since it's effectively the same implementation as the suffix-finding code for the docker engine version, but hopefully these can all get a refactor into a common function later ?

Yep, this is alright for now, I can refactor it when I remove the default value.
Seems like the tests are passing 🎉

Originally posted by @samruddhikhandale in #838 (comment)