Support code gen using ? for wildcards

[lacarvalho91]

In Scala 3 it is possible to use _ for type lambda placeholders by setting the -Ykind-projector:underscores compiler flag. When this is enabled _ can no longer be used for wildcards, ? must be used instead. The code gen doesn't support this ATM so you end up with compiler errors from the generated code.

[ahjohannessen]

ScalaPB needs to support this as fs2-grpc piggy-backs on it. See scalapb/ScalaPB#1316

Changing the code generator is doable, but until ScalaPB supports it we have to wait to implement it. I have a PoC that feeds the code generator with scala target version allowing it select different code paths depending on that value.