In the past we had a special function to do mb_encode_mimeheader correctly.
Since PHP 8.2 this works perfectly fine, so all the code was removed and
replaced with just the normal "mb_encode_mimeheader" call with the same
settings as before:
- set global encoding to parameter
- run encoding with charset, 'B' for transfer and use the line break given in parameter
- reset the global encoding to previous set
