Change NET6_0_OR_GREATER to NET8_0_OR_GREATER

Co-authored-by: adamhathcock <527620+adamhathcock@users.noreply.github.com>

src/SharpCompress/Common/AsyncBinaryReader.cs

@@ -82,7 +82,7 @@ namespace SharpCompress.Common
             }
         }
 
-#if NET6_0_OR_GREATER
+#if NET8_0_OR_GREATER
         public async ValueTask DisposeAsync()
         {
             if (_disposed)

src/SharpCompress/Compressors/ZStandard/UnsafeHelper.cs

@@ -16,7 +16,7 @@ public static unsafe class UnsafeHelper
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static void* malloc(ulong size)
     {
-#if NET6_0_OR_GREATER
+#if NET8_0_OR_GREATER
         var ptr = NativeMemory.Alloc((nuint)size);
 #else
         var ptr = (void*)Marshal.AllocHGlobal((nint)size);
@@ -31,7 +31,7 @@ public static unsafe class UnsafeHelper
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static void* calloc(ulong num, ulong size)
     {
-#if NET6_0_OR_GREATER
+#if NET8_0_OR_GREATER
         return NativeMemory.AllocZeroed((nuint)num, (nuint)size);
 #else
         var total = num * size;
@@ -53,7 +53,7 @@ public static unsafe class UnsafeHelper
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public static void free(void* ptr)
     {
-#if NET6_0_OR_GREATER
+#if NET8_0_OR_GREATER
         NativeMemory.Free(ptr);
 #else
         Marshal.FreeHGlobal((IntPtr)ptr);

src/SharpCompress/Utility.cs

@@ -189,7 +189,7 @@ internal static class Utility
             }
         }
 
-#if NET6_0_OR_GREATER
+#if NET8_0_OR_GREATER
         public bool ReadFully(byte[] buffer)
         {
             try