Add Scala compiler plugin "sbt-silencer"

Motivation:

Add "deprecated" warnings for the users, but avoid compilation errors
when used internally for the period of migration.

Modification:

- Add sbt silencer to `build.sbt`
- Deprecate `MVar` and promote `MVar2`

build.sbt

@@ -28,6 +28,7 @@ val CompileTime = config("CompileTime").hide
 val SimulacrumVersion = "1.0.0"
 val CatsVersion = "2.1.0"
 val DisciplineScalatestVersion = "1.0.0"
+val SilencerVersion = "1.4.4"
 val customScalaJSVersion = Option(System.getenv("SCALAJS_VERSION"))
 
 addCommandAlias("ci", ";scalafmtSbtCheck ;scalafmtCheckAll ;test ;mimaReportBinaryIssues; doc")
@@ -235,6 +236,10 @@ lazy val core = crossProject(JSPlatform, JVMPlatform)
       "org.typelevel" %%% "cats-laws" % CatsVersion % Test,
       "org.typelevel" %%% "discipline-scalatest" % DisciplineScalatestVersion % Test
     ),
+    libraryDependencies ++= Seq(
+      compilerPlugin(("com.github.ghik" % "silencer-plugin" % SilencerVersion).cross(CrossVersion.full)),
+      ("com.github.ghik" % "silencer-lib" % SilencerVersion % Provided).cross(CrossVersion.full)
+    ),
     libraryDependencies ++= {
       CrossVersion.partialVersion(scalaVersion.value) match {
         case Some((2, v)) if v <= 12 =>

core/shared/src/main/scala/cats/effect/concurrent/MVar.scala

@@ -20,6 +20,7 @@ package concurrent
 import cats.effect.concurrent.MVar.{TransformedMVar, TransformedMVar2}
 import cats.effect.internals.{MVarAsync, MVarConcurrent}
 import cats.~>
+import com.github.ghik.silencer.silent
 
 /**
  * A mutable location, that is either empty or contains
@@ -52,10 +53,10 @@ import cats.~>
  *
  * Inspired by `Control.Concurrent.MVar` from Haskell and
  * by `scalaz.concurrent.MVar`.
- *
- * Note: the interface `MVar` is now deprecated in favour of a new generation `MVar2` that supports `tryRead`.
  */
+@deprecated("`MVar` is now deprecated in favour of a new generation `MVar2` with `tryRead` support", "2.1.0")
 abstract class MVar[F[_], A] {
+
   /**
    * Returns `true` if the `MVar` is empty and can receive a `put`, or
    * `false` otherwise.
@@ -126,7 +127,9 @@ abstract class MVar[F[_], A] {
  * The `MVar2` is the successor of `MVar` with [[tryRead]]. It was implemented separately only to maintain binary
  * compatibility with `MVar`.
  */
+@silent("deprecated")
 abstract class MVar2[F[_], A] extends MVar[F, A] {
+
   /**
    * Returns the value without waiting or modifying.
    *
@@ -145,6 +148,7 @@ abstract class MVar2[F[_], A] extends MVar[F, A] {
 
 /** Builders for [[MVar]]. */
 object MVar {
+
   /**
    * Builds an [[MVar]] value for `F` data types that are [[Concurrent]].
    *
@@ -257,6 +261,7 @@ object MVar {
    * Returned by the [[apply]] builder.
    */
   final class ApplyBuilders[F[_]](val F: Concurrent[F]) extends AnyVal {
+
     /**
      * Builds an `MVar` with an initial value.
      *