Deprecate PatternMatch (#4661)

Author: ismaelhamed

src/core/Akka.API.Tests/CoreAPISpec.ApproveCore.approved.txt

@@ -2111,6 +2111,7 @@ namespace Akka
         public override int GetHashCode() { }
         public override string ToString() { }
     }
+    [System.ObsoleteAttribute("Use instead the pattern matching feature introduced in C# 7.0")]
     public class static PatternMatch
     {
         public static Akka.Case Match(this object target) { }

src/core/Akka/PatternMatch.cs

@@ -12,6 +12,7 @@ namespace Akka
     /// <summary>
     /// Class PatternMatch.
     /// </summary>
+    [Obsolete("Use instead the pattern matching feature introduced in C# 7.0")]
     public static class PatternMatch
     {