fix UnusedScalafixSuppression warnings on Scala 3 scalafix invocations

Since ExplicitResultTypes is unavailable in Scala 3, some silencers are
rightfully detected as unused for that invocation. Qualifying these silencers
by suffixing ExplicitResultTypes does not help, since EscapeHatch is not aware
of which rules are executed, so we need to address the problem differently.

- For the (v0) signature, this removes the silencer to be explicit about the
  type since there is no reason (except compatibility) to keep it this way.
  Since this is the old API and a very specific class, it's unlikely that will
  will cause linking errors when running community rules with existing
  versions.
- For test suites where we DO want an explicit type, this turns private the
  terms that ExplicitResultTypes warns on to stop triggering it. However, this
  triggers RemoveUnused, but since it is now common to both Scala 2 and Scala 3
  and, it does not trigger UnusedScalafixSuppression in any invocation.

project/Mima.scala

@@ -9,7 +9,7 @@ object Mima {
       ProblemFilters.exclude[Problem]("scalafix.internal.*"),
       ProblemFilters.exclude[Problem]("scala.meta.internal.*"),
       // Exceptions
-      ProblemFilters.exclude[DirectMissingMethodProblem]("scalafix.Versions.scala211")
+      ProblemFilters.exclude[DirectMissingMethodProblem]("scalafix.v0.Signature#Self.syntax")
     )
   }
 }

scalafix-core/src/main/scala/scalafix/v0/Signature.scala

@@ -50,10 +50,8 @@ object Signature {
   }
 
   final case class Self(name: String) extends Signature {
-    override def syntax =
-      throw new UnsupportedOperationException(
-        "No longer supported."
-      ) // scalafix:ok
+    override def syntax: String =
+      throw new UnsupportedOperationException("No longer supported.")
     override def structure: String = s"""Signature.Self("$name")"""
     override def toString: String = syntax
   }

scalafix-tests/integration/src/main/scala/test/DenotationOpsTest.scala

@@ -4,5 +4,5 @@ package test
 object DenotationOpsTest {
   def m(x: Int, y: String): List[String] = List(y)
   var x = true
-  val y = m(42, "hey")
+  private val y = m(42, "hey")
 }

scalafix-tests/integration/src/main/scala/test/TypesTest.scala

@@ -3,11 +3,11 @@ package test
 
 class TypesTest {
   val a = 42
-  val b = List(42)
+  private val b = List(42)
   class Inner
   val c = new TypesTest
   val d = new c.Inner
-  val e = null.asInstanceOf[TypesTest#Inner]
+  private val e = null.asInstanceOf[TypesTest#Inner]
   val f: {
     def foo(a: Int): Int
     def bar(a: Int): Int