PHP 8 compatibility

[indigoxela]

Some issues have been fixed in D7, but there's more.

This issue here combines several PHP 8 compat tasks.

[indigoxela]

PHPStan finds a lot more to nag, but here we address the "undefined property" issues:

• Access to an undefined property PHPMailer::$is_html.
• Access to an undefined property PHPMailer::$DKIM_passphrase
• Access to an undefined property PHPMailerOAuth::$AuthType
• Access to an undefined property PHPMailerOAuth::$SMTPOptions

They're possible not used often, but finding these violations is the benefit of static analysis. :wink:

[herbdool]

The PR looks good. RTBC.

I'll add that it also removes reference to safe_mode which was removed in PHP 5.4.

[yorkshire-pudding]

I also confirm this works. Please can this be merged and released @jenlampton