feat(manager/sbt): support `ThisBuild` scoped `scalaVersion` (#13544)

* fix(manager/sbt): support `ThisBuild` scoped `scalaVersion`

Using `ThisBuild / scalaVersion` to specify a common `scalaVersion`
across subprojects is a well-known technique such as described in
the official documentation of SBT.

<https://www.scala-sbt.org/1.x/docs/Scopes.html#Build-level+settings

>

So, we should support to recognize this syntax also.

* fix(manager/sbt): support `ThisBuild / scalaVersion := variable`

Co-authored-by: Rhys Arkins <rhys@arkins.net>