Add Scala 3 support

[danicheg]

Because finagle is not published for Scala 3 we are using the for3Use2_13 option.

[danicheg]

Though util-core is published for Scala 3, finagle is dependent on it, so we were forced to use the for3Use2_13 option 🤷🏻‍♂️

[felixbr]

I'm not sure we should publish an artifact using the for3Use2_13 option. It's not meant for libraries as it easily creates dependency hell if someone depends on other cats-libraries with proper Scala 3 support.

Instead you should depend on catbird using the for3Use2_13 option in your application if that's possible with twitter libraries.

edit: I'm not at all opposed to changes that will make a future Scala 3 port easier or ones that allow for3Use2_13 to work in applications

edit2: See also point 5 in Daniel Spiewak's guidelines on Scala 3 cross-publishing

[danicheg]

@felixbr your position is reasonable. So that PR will be waiting for the release finagle on Scala 3, I'm ok with it.

[bpholt]

I'm going to close this for now, but please feel free to reopen once Finagle supports Scala 3.