Bump SBT to 1.8.0 (#2870)

This means dropping Scala 2.12.4 support which is broken in Zinc newer
than 1.6.0. Chisel users of Scala 2.12.4 should upgrade to 2.12.17.

Author: jackkoenig

build.sbt

@@ -12,10 +12,8 @@ val defaultVersions = Map(
 )
 
 lazy val commonSettings = Seq(
-  resolvers ++= Seq(
-    Resolver.sonatypeRepo("snapshots"),
-    Resolver.sonatypeRepo("releases")
-  ),
+  resolvers ++= Resolver.sonatypeOssRepos("snapshots"),
+  resolvers ++= Resolver.sonatypeOssRepos("releases"),
   organization := "edu.berkeley.cs",
   version := "3.6-SNAPSHOT",
   autoAPIMappings := true,
@@ -94,7 +92,7 @@ lazy val pluginScalaVersions = Seq(
   // scalamacros paradise version used is not published for 2.12.0 and 2.12.1
   "2.12.2",
   "2.12.3",
-  "2.12.4",
+  // 2.12.4 is broken in newer versions of Zinc: https://github.com/sbt/sbt/issues/6838
   "2.12.5",
   "2.12.6",
   "2.12.7",

project/build.properties

@@ -1 +1 @@
-sbt.version=1.5.8
+sbt.version=1.8.0