Do not hardcode sbt-scalafix version

[fthomas]

We currently hardcode the version of sbt-scalafix that we add to the global sbt plugins temporarily while applying Scalafix migrations:

scala-steward/modules/core/src/main/scala/org/scalasteward/core/buildtool/sbt/package.scala

Line 41 in cc04fcf

"""addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.32")"""

This is not ideal for Scala Steward instances that do not track the latest changes from this repository. If a new sbt-scalafix version is released that supports newer Scala versions, Scala Steward instances that not yet include that latest sbt-scalafix version might not successfully apply Scalafix migrations in projects that are already on the new Scala version. I think it would be better to use the latest sbt-scalafix version from the VersionsCache.

[bjaglin]

With the upcoming scalacenter/sbt-scalafix#292, scalafixEnable will be able to work with future Scala versions (i.e. published after the release of the sbt-scalafix running). I am wondering if it's not better to revert this change for the sake of a more deterministic behavior?