Remove support for scala 2.12

Main reason to keep was to be compatible with Databricks
But Databricks Runtime 16.3 should come early March and have support for 2.13&3
https://community.databricks.com/t5/data-engineering/when-will-db-release-runtime-with-scala-2-13/td-p/104399
https://docs.databricks.com/en/release-notes/runtime/index.html

Note: not yet cleaning up code that could move to newer constructs not supported by 2.12

Author: dmivankov

build.sbt

@@ -3,9 +3,8 @@ import sbtassembly.MergeStrategy
 import scala.xml.{Node => XmlNode, NodeSeq => XmlNodeSeq, _}
 import scala.xml.transform.{RewriteRule, RuleTransformer}
 
-val scala212 = "2.12.19"
 val scala213 = "2.13.15"
-val supportedScalaVersions = List(scala212, scala213)
+val supportedScalaVersions = List(scala213)
 val sparkVersion = "3.3.4"
 val circeVersion = "0.14.9"
 val sttpVersion = "3.5.2"
@@ -46,7 +45,7 @@ lazy val commonSettings = Seq(
   crossScalaVersions := supportedScalaVersions,
   semanticdbEnabled := true,
   semanticdbVersion := scalafixSemanticdb.revision,
-  scalaVersion := scala212, // default to Scala 2.12
+  scalaVersion := scala213, // default to Scala 2.13
   // handle cross plugin https://github.com/stringbean/sbt-dependency-lock/issues/13
   dependencyLockFile := { baseDirectory.value / s"build.scala-${CrossVersion.partialVersion(scalaVersion.value) match { case Some((2, n)) => s"2.$n" }}.sbt.lock" },
   description := "Spark data source for the Cognite Data Platform.",
@@ -60,9 +59,6 @@ lazy val commonSettings = Seq(
       "-Wconf:src=src/test/scala/.*&cat=other-pure-statement:i",
       "-Wconf:src=src/test/scala/.*&msg=unused value of type org.scalatest.Assertion:s"
     )
-    case Some((2, 12)) => Seq(
-      "-Wconf:src=src/test/scala/.*&cat=unused:i"
-    )
     case _ => Seq.empty
   }),
   resolvers ++= Resolver.sonatypeOssRepos("releases"),
@@ -158,7 +154,6 @@ lazy val library = (project in file("."))
       "io.scalaland" %% "chimney" % "1.1.0"
         // scala-collection-compat is used in stdlib collections conversion,
         // and this dependency causes issues with Livy.
-        exclude("org.scala-lang.modules", "scala-collection-compat_2.12")
         exclude("org.scala-lang.modules", "scala-collection-compat_2.13"),
       "org.specs2" %% "specs2-core" % Specs2Version % Test,
       "com.softwaremill.sttp.client3" %% "async-http-client-backend-fs2" % sttpVersion,
@@ -167,18 +162,13 @@ lazy val library = (project in file("."))
         exclude("io.netty", "netty-buffer")
         exclude("io.netty", "netty-handler")
         exclude("io.netty", "netty-transport-native-epoll")
-        exclude("com.softwaremill.sttp", "circe_2.12")
         exclude("com.softwaremill.sttp", "circe_2.13")
-        exclude("org.typelevel", "cats-effect_2.12")
         exclude("org.typelevel", "cats-effect_2.13")
-        exclude("org.typelevel", "cats-core_2.12")
         exclude("org.typelevel", "cats-core_2.13"),
       "org.slf4j" % "slf4j-api" % "2.0.9" % Provided,
       "io.circe" %% "circe-generic" % circeVersion
-        exclude("org.typelevel", "cats-core_2.12")
         exclude("org.typelevel", "cats-core_2.13"),
       "io.circe" %% "circe-generic-extras" % "0.14.3"
-        exclude("org.typelevel", "cats-core_2.12")
         exclude("org.typelevel", "cats-core_2.13"),
       "org.scalatest" %% "scalatest" % "3.0.8" % Test,
       "org.eclipse.jetty" % "jetty-servlet" % "9.4.44.v20210927" % Provided,