Remove old frameworks

tests/ImageSharp.Benchmarks/Config.cs

@@ -32,17 +32,13 @@ namespace SixLabors.ImageSharp.Benchmarks
         public class MultiFramework : Config
         {
             public MultiFramework() => this.AddJob(
-                    Job.Default.WithRuntime(ClrRuntime.Net472),
-                    Job.Default.WithRuntime(CoreRuntime.Core31),
-                    Job.Default.WithRuntime(CoreRuntime.Core50).WithArguments(new Argument[] { new MsBuildArgument("/p:DebugType=portable") }));
+                    Job.Default.WithRuntime(CoreRuntime.Core60).WithArguments(new Argument[] { new MsBuildArgument("/p:DebugType=portable") }));
         }
 
         public class ShortMultiFramework : Config
         {
             public ShortMultiFramework() => this.AddJob(
-                    Job.Default.WithRuntime(ClrRuntime.Net472).WithLaunchCount(1).WithWarmupCount(3).WithIterationCount(3),
-                    Job.Default.WithRuntime(CoreRuntime.Core31).WithLaunchCount(1).WithWarmupCount(3).WithIterationCount(3),
-                    Job.Default.WithRuntime(CoreRuntime.Core50).WithLaunchCount(1).WithWarmupCount(3).WithIterationCount(3).WithArguments(new Argument[] { new MsBuildArgument("/p:DebugType=portable") }));
+                    Job.Default.WithRuntime(CoreRuntime.Core60).WithLaunchCount(1).WithWarmupCount(3).WithIterationCount(3).WithArguments(new Argument[] { new MsBuildArgument("/p:DebugType=portable") }));
         }
 
         public class ShortCore31 : Config