Upgrade PHPMailer to 5.2.8

This implements the HTML5-spec email validation pattern, which is what
we will be using with validateAddress() as per discussion in pull
request https://github.com/mantisbt/mantisbt/pull/172.

See changelog.md for full details
Add HTML5 email validation method (as default)

As per grangeway's suggestion in pull request
https://github.com/mantisbt/mantisbt/pull/172, we are now using an HTML5
specification-compliant regex to validate e-mail addresses by default
instead of PHP method.

Issue #16894