Swift 5.2 minimum; bugfix in base64 auth; fix CI

Misc dependency update (#125)

* Update dependencies; Swift 5.2 is now the minimum supported version.

* Remove Swift 4.x support

* Bugfix: base64 auth should not have newline; support SSL and non-SSL ports for tests

- base64 encoding auth should not have line length limit
- Tests need to support SSL/TLS and non-SSL ports for mail servers that
  treat them differently

* bugfix: data type for cache

* Update CI: Xcode 13.2; New way to specify development snapshot