Add Scala 3.6 to build coverage

[djspiewak]

Doesn't necessarily need to be the full matrix explosion so long as we're still publishing on 3.3, but as we're a part of the community build it would be helpful to ensure we don't regress the changes instituted in #4381.

[armanbilge]

Alternatively, could Scala 3.3 LTS add a compiler flag that lets this rise to a fatal warning? Enabling that flag in sbt-typelevel seems more feasible than adding a 3.6 cross-build to every repository. cc @mbovel

[mbovel]

I don't know. One might be able to add warnings in LTS for specific things, for example for requiring using for implicit arguments. But we cannot generally emulate the complete Scala Next behavior in LTS.