Some error messages in PHP8 changed from notice->warning or from warning->error, some error texts also slightly changed wordings. It made some unittests fail

This issue:

1. addresses a change in PHP 8.0 to standardise all type warning messages: https://github.com/php/php-src/pull/5092
2. addresses a change in PHP 8.0 whereby some notices have been converted to warnings in preparation for deprecation of legacy behaviours (documented at https://www.php.net/manual/en/migration80.incompatible.php)
3. allows for a change in expected behaviour whereby Mail headers are now consistently wrapped with CRLF instead of LF

For the change in Mail headers, this change relates to a documented change in PHP, and adoption of the same change in PHPMailer. It is the change to PHPMailer which affects us here.