Fix IDE0057 warnings

Fix IDE0057 warnings identified in #2003.

src/Polly.Core/Utils/TypeNameFormatter.cs

@@ -17,6 +17,12 @@ public static string Format(Type type)
             return type.Name;
         }
 
-        return $"{type.Name.Substring(0, type.Name.Length - GenericSuffixLength)}<{Format(args[0])}>";
+#if NET6_0_OR_GREATER
+        var nameNoAirity = type.Name[..(type.Name.Length - GenericSuffixLength)];
+#else
+        var nameNoAirity = type.Name.Substring(0, type.Name.Length - GenericSuffixLength);
+#endif
+
+        return $"{nameNoAirity}<{Format(args[0])}>";
     }
 }

src/Polly/Utilities/KeyHelper.cs

@@ -6,5 +6,9 @@ internal static class KeyHelper
     private const int GuidPartLength = 8;
 
     public static string GuidPart() =>
+#if NET6_0_OR_GREATER
+        Guid.NewGuid().ToString()[..GuidPartLength];
+#else
         Guid.NewGuid().ToString().Substring(0, GuidPartLength);
+#endif
 }