Fix regression in RegexGenerator.GetSHA256FieldName (#101606)

A previous refactoring deleted the use of the hash.

Author: stephentoub

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

@@ -5405,7 +5405,7 @@ private static string GetSHA256FieldName(string prefix, string toEncode)
         {
 #pragma warning disable CA1850 // SHA256.HashData isn't available on netstandard2.0
             using SHA256 sha = SHA256.Create();
-            return $"{prefix}{ToHexStringNoDashes(Encoding.UTF8.GetBytes(toEncode))}";
+            return $"{prefix}{ToHexStringNoDashes(sha.ComputeHash(Encoding.UTF8.GetBytes(toEncode)))}";
 #pragma warning restore CA1850
         }