Unify CharToHexLookup as ReadOnlySpan<byte>

Now that Span<T>/ReadOnlySpan<T> live in the System namespace on all TFMs,
the RVA-backed `static ReadOnlySpan<byte> Prop => new byte[] {...}` idiom
compiles correctly on net461/netstandard2.0 too. Collapse the NETCOREAPP /
net461 ifdef around CharToHexLookup into a single ReadOnlySpan<byte> property
and drop the now-unused SA1201 pragma block.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: andrewlock

tracer/src/Datadog.Trace/Util/HexConverter.cs

@@ -35,13 +35,7 @@ public enum Casing : uint
     /// <summary>
     /// Gets a map from an ASCII char to its hex value, e.g. arr['b'] == 11. 0xFF means it's not a hex digit.
     /// </summary>
-#if NETCOREAPP
     private static ReadOnlySpan<byte> CharToHexLookup =>
-#else
-#pragma warning disable SA1201 // Elements should appear in the correct order
-    private static readonly byte[] CharToHexLookup =
-#pragma warning restore SA1201 // Elements should appear in the correct order
-#endif
     [
         0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 15
         0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, // 31