build: Prepare Scala 3 build

[jakubjanecek]

Preparation for Scala 3 build.

We are missing stable build of http4s for Scala 3 and we are missing build of PureConfig with automatic derivations (currently the supporrt is only experimental).

There are still some issues in Scala 3 compilation which need to be solved.

[jakubjanecek]

@jendakol @sideeffffect I realized that we could probably try to finish this PR. PureConfig already has support for Scala 3 but not for ConfigWriter so I decided to remove those few ConfigWriter instances we have (I don't want them to block this PR, their not really needed for SST, they are a useful thing for someone who wants to write out configuration somewhere, that can be done in the app directly if needed).

But the problem is that PureConfig does not want to derive ConfigReader instances for me and I don't really know how to solve it. Do you have any experience with this? Could you have a look at it?

It's not really documented anywhere but the only thing I can think of is that maybe we will need to split the code and have different derivation for 2.x and 3.x but I've never done that before.

[tomasherman]

replaced by #872