Require Swift 6

[ktoso]

Due to some memory leak issues in remote calls that only got fixed in Swift 6 we are considering to require the use of Swift 6 in this library.

Usage is relatively low, and all adopters I'm aware of are okey to update their Swift dependency (or already have).

If someone with production use has concerns with this please let us know.

[akbashev]

Should be fixed now