Deprecate the Build trait

main/src/main/scala/sbt/Build.scala

@@ -8,6 +8,7 @@ import Keys.{ name, organization, thisProject, autoGeneratedProject }
 import Def.{ ScopedKey, Setting }
 
 // name is more like BuildDefinition, but that is too long
+@deprecated("Use .sbt format instead", "0.13.12")
 trait Build {
   def projectDefinitions(baseDirectory: File): Seq[Project] = projects
   def projects: Seq[Project] = ReflectUtilities.allVals[Project](this).values.toSeq

notes/0.13.12/deprecate-Build.markdown

@@ -0,0 +1,10 @@
+  [@dwijnand]: https://github.com/dwijnand
+  [#2530]: https://github.com/sbt/sbt/pull/2530
+
+### Fixes with compatibility implications
+
+- The Build trait is deprecated in favour of the .sbt format [#2530][#2530] by [@dwijnand][@dwijnand]
+
+### Improvements
+
+### Bug fixes