PHPMailer 6.5.1

This is a maintenance release.

• Provisional support for PHP 8.1
• Major overhaul of test suite
• Add codecov.io coverage reporting
• Prefer implicit TLS on port 465 as default encryption scheme in examples, as per RFC8314
• Fix potential noisy output from IMAP address parser
• Stricter checking of custom MessageID validity
• Replace invalid default From address
• Support fallback for languages, so a request for pt_xx will fall back to pt rather than the default en.
• Support multi-line RFC2047-encoded addresses in parseAddresses
• Improved Japanese translation

Many thanks to @jrfnl for all her work.