Flagger should not silently pick a default mesh router if the requested one doesn't match

[mtfurlan]

Describe the feature

I just spent quite a long time not noticing that "gatewayapi" and "apigateway" are different.
But flagger was doing something weird with istio

I finally tracked it down to the router factory silently picking a default here.

Proposed solution

I think that flagger should log a warning that "provider %s was requested, but Istio was used"

Any alternatives you've considered?

Flagger could refuse to pick a default and just fail, which I would prefer, but that seems more of a breaking change than just a warning in the log.