mail/phpmailer6: Update to 6.7

Changelog:
-Break out boundary definitions into a method (note that boundary format has also changed slightly)
-Remove MIME preamble to match popular client behaviour, may help with DKIM too
-Fix handling of trailing whitespace in simple DKIM canonicalisation
-Fix some possible POP3 auth issues, including a TCP hang (thanks to @czirkoszoltan)
-Add Azure XOAUTH2 example and docs (thanks to @greew)
-Preserve errors during disconnect
-Avoid some PHP 8.1 type issues
-Update CI to run on Ubuntu 22.04

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