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

@@ -29,6 +29,7 @@ val CompileTime = config("CompileTime").hide
 val SimulacrumVersion = "1.0.0"
 val CatsVersion = "2.1.1"
 val DisciplineScalatestVersion = "1.0.1"
+val SilencerVersion = "1.6.0"
 val customScalaJSVersion = Option(System.getenv("SCALAJS_VERSION"))
 
 addCommandAlias("ci", ";scalafmtSbtCheck ;scalafmtCheckAll ;test ;mimaReportBinaryIssues; doc")
@@ -239,6 +240,11 @@ 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 % CompileTime).cross(CrossVersion.full),
+      ("com.github.ghik" % "silencer-lib" % SilencerVersion % Test).cross(CrossVersion.full)
+    ),
     libraryDependencies ++= {
       CrossVersion.partialVersion(scalaVersion.value) match {
         case Some((2, v)) if v <= 12 =>
@@ -330,9 +336,6 @@ lazy val siteSettings = Seq(
       Map("permalink" -> "/", "title" -> "Home", "section" -> "home", "position" -> "0")
     )
   ),
-  micrositeConfigYaml := ConfigYml(
-    yamlCustomProperties = Map("plugins" -> List("jekyll-relative-links"))
-  ),
   micrositeCompilingDocsTool := WithMdoc,
   mdocIn := (sourceDirectory in Compile).value / "mdoc",
   fork in mdoc := true,

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
 
 /**
  * @define mVarDescription A mutable location, that is either empty or contains a value of type `A`.
@@ -58,7 +59,9 @@ sealed private[concurrent] trait MVarDocumentation extends Any {}
 /**
  * $mVarDescription
  */
+@deprecated("`MVar` is now deprecated in favour of a new generation `MVar2` with `tryRead` and `swap` support", "2.2.0")
 abstract class MVar[F[_], A] extends MVarDocumentation {
+
   /**
    * Returns `true` if the `MVar` is empty and can receive a `put`, or
    * `false` otherwise.
@@ -131,7 +134,9 @@ abstract class MVar[F[_], A] extends MVarDocumentation {
  * The `MVar2` is the successor of `MVar` with [[tryRead]] and [[swap]]. It was implemented separately only to maintain
  * binary compatibility with `MVar`.
  */
+@silent("deprecated")
 abstract class MVar2[F[_], A] extends MVar[F, A] {
+
   /**
    * Replaces a value in MVar and returns the old value.
 
@@ -158,6 +163,7 @@ 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]].
    *