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

5.1.200

Updated dependencies for change in github org.

5.1.2

• Resolve compilation warnings with Swift 5 and later

Swift 5 support

Merge pull request #99 from IBM-Swift/swift5

Support Swift 5

Swift 4.1.2 support

Thanks @pushkarnk

Add TLSMode

Thanks to @trametheka for the great work.

migration guide

Update dependencies

Fixes #74. Thanks @Davidde94

4.0.0

Adds Swift 4.1 support and more. See all changes in migration guide.

Thanks for contributions from @hanskroner & @rafiki270.

Specify TLS support

Migration guide

Use 'text/plain' for plain text content

Thanks @hanskroner