Change saved song file encoding to UTF-8 without BOM

Previously the encoding was System.Text.Encoding.UTF8, which produces a BOM at the beginning of the saved file. Now it is a new System.Text.UTF8Encoding, which does not produce a BOM.

UltraStar Play/Packages/playshared/Runtime/Song/UltraStarSongFileWriter.cs

@@ -9,7 +9,8 @@ public static class UltraStarSongFileWriter
     public static void WriteFile(string absolutePath, SongMeta songMeta)
     {
         string txtFileContent = GetTxtFileContent(songMeta);
-        File.WriteAllText(absolutePath, txtFileContent, Encoding.UTF8);
+        // Do not use Encoding.UTF8 as it will produce a BOM.
+        File.WriteAllText(absolutePath, txtFileContent, new UTF8Encoding());
     }
 
     private static string GetTxtFileContent(SongMeta songMeta)