scrooge: Fix sbt scala version misalignment

Problem

Scrooge sbt build is broken, preventing us from releasing CSL libraries.

There were two problems with the build
 - Scala versions for 2.12 and 2.13 were misaligned between individual projects
 - The top-level project scrooge (only used in testing the sbt build) had its own settings that were shadowing individual projects settings,
   so the bug with the misaligned version slipped through. We only noticed an issue during a release (when we release individual projects).

Solution

Fix the build by making sbt happy.

Differential Revision: https://phabricator.twitter.biz/D942948

build.sbt

@@ -186,11 +186,13 @@ val settingsWithTwoTen =
       )
     )
 
+val commonScalaVersions = Seq("2.12.12", "2.13.6")
+
 // settings for projects that are cross compiled with scala 2.10
 val settingsCrossCompiledWithTwoTen =
   sharedSettingsWithoutScalaVersion ++
     Seq(
-      crossScalaVersions := Seq("2.10.7", "2.12.12", "2.13.1"),
+      crossScalaVersions := Seq("2.10.7") ++ commonScalaVersions,
       scalaVersion := "2.13.6",
       scalacOptions := scalacTwoTenOptions,
       javacOptions ++= Seq("-source", "1.8", "-target", "1.8", "-Xlint:unchecked"),
@@ -204,7 +206,7 @@ val sharedSettings =
   sharedSettingsWithoutScalaVersion ++
     Seq(
       scalaVersion := "2.13.6",
-      crossScalaVersions := Seq("2.12.8", "2.13.6"),
+      crossScalaVersions := commonScalaVersions,
       scalacOptions := Seq(
         "-deprecation",
         "-unchecked",
@@ -244,8 +246,6 @@ lazy val scrooge = Project(
   base = file(".")
 ).enablePlugins(
   ScalaUnidocPlugin
-).settings(
-  sharedSettings
 ).aggregate(publishedProjects: _*)
 
 // This target is used for publishing dependencies locally
@@ -260,8 +260,6 @@ lazy val scroogePublishLocal = Project(
   // use a different target so that we don't have conflicting output paths
   // between this and the `scrooge` target.
   base = file("target/")
-).settings(
-  sharedSettings
 ).aggregate(publishedProjects: _*)
 
 // must be cross compiled with scala 2.10 because scrooge-sbt-plugin