Stack has not been tested with GHC & Cabal versions

This is just a compatibility warning. The next stack version should be tested with GHC 8.8 and this warning should be gone. Until then there's probably little you can do unless you want to modify the source.

As the others said, this can be most likely be ignored. If you want to be on the super-safe side, edit your stack.yaml and downgrade the resolver back to 14.x (latest at this point is 14.27).