Flow for converting "upgraded expression" warnings into errors (or vice versa)

[dylant-da]

As per https://github.com/digital-asset/daml/issues/17632#issuecomment-1775723874, we should be able to convert warnings into errors or vice versa for expression upgrades.

I think flags would be a good approach, with a whitelisting mechanism as well. Should go over user benefits and flow - possibly discuss with product.