Clean up, drop System.Net.Mail dependency, update to MailKit 4.3

[nblumhardt]

This completely drops the System.Net.Mail dependency; users will need to migrate their configurations to be compatible with MailKit, including explicitly choosing a SecureSocketOptions value, and setting up any necessary credentials.

Edit: a few more things; this is full of breaking changes:

• The WriteTo.Email() overloads have changed: a configuration-friendly inline one, and one that accepts complete connection/batching/formatting implementations
• Obsolete TFMs may not be well-supported - various dependencies are not tested with e.g. net5.0 and earlier
• As there's no XML email settings support (that I'm aware of) in MailKit, the SMTP host must now always be specified

[nblumhardt]

Merging; will get some feedback on this, hopefully resolving the major issues around MailKit versions, still keen for feedback but need to push ahead while I have some holiday hours spare to sort this out :-)