mail/phpmailer: Deprecate and set expiration date

Upstream states that PHPMailer 5.2 is no longer supported, even for security
updates. The last release v5.2.28 and the final commit on the 5.2-stable
branch both date from March 2020. Switch to mail/phpmailer6 or
mail/phpmailer7, both of which are maintained.

MFH: 2026Q3
Sponsored by: Netzkommune GmbH