Bump NETCOREAPP3_0_OR_GREATER to NET6_0_OR_GREATER (#4367)

Author: alanwest

src/OpenTelemetry.Api/Internal/Guard.cs

@@ -42,7 +42,7 @@ public CallerArgumentExpressionAttribute(string parameterName)
 }
 #endif
 
-#if !NETCOREAPP3_0_OR_GREATER && !NETSTANDARD2_1_OR_GREATER
+#if !NET6_0_OR_GREATER && !NETSTANDARD2_1_OR_GREATER
 namespace System.Diagnostics.CodeAnalysis
 {
     /// <summary>Specifies that an output is not <see langword="null"/> even if

src/OpenTelemetry/Internal/MathHelper.cs

@@ -15,7 +15,7 @@
 // </copyright>
 
 using System.Diagnostics;
-#if NETCOREAPP3_0_OR_GREATER
+#if NET6_0_OR_GREATER
 using System.Numerics;
 #endif
 using System.Runtime.CompilerServices;
@@ -86,7 +86,7 @@ public static int LeadingZero32(int value)
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static int LeadingZero64(long value)
     {
-#if NETCOREAPP3_0_OR_GREATER
+#if NET6_0_OR_GREATER
         return BitOperations.LeadingZeroCount((ulong)value);
 #else
         unchecked