[2.x] use `ClassTag` instead of deprecated `scala.reflect.Manifest`

[xuwei-k]

io/io/src/main/scala/sbt/io/IO.scala

Line 34 in 53f95d7

import scala.reflect.{ Manifest => SManifest }

I think we should replace to ClassTag before sbt 2.0 final release. But maybe it break binary compatibility.

This repo does not have separate branches for sbt 1.x and 2.x. create new branch? or create src/main/scala-3 and then change only Scala 3 sources?

[eed3si9n]

Currently io is same for both sbt 1.x and 2.x but we could branch to develop and 1.x branch like other repos? /cc @adpi2