Fix TestUnicodeUtil: Use IOUtils.ENCODING_UTF_8_NO_BOM instead of non-existent CHARSET_UTF_8

Author: paulirwin

src/Lucene.Net.Tests/Util/TestUnicodeUtil.cs

@@ -353,7 +353,7 @@ public void TestUTF8toUTF16Exception(byte[] invalidUtf8, bool shouldThrow)
         public void TestTryUTF8toUTF16()
         {
             string unicode = TestUtil.RandomRealisticUnicodeString(Random);
-            var utf8 = new BytesRef(IOUtils.CHARSET_UTF_8.GetBytes(unicode));
+            var utf8 = new BytesRef(IOUtils.ENCODING_UTF_8_NO_BOM.GetBytes(unicode));
 
             bool success = UnicodeUtil.TryUTF8toUTF16(utf8, out var chars);