[Konversation] Port from deprecated KTcpSocket to QSslSocket

Summary:
Increase the min. required Qt version to 5.7.0 since that was the version
where QOverload was introduced.

Reviewers: Konversation, dvratil, vkrause

Reviewed By: dvratil

Subscribers: kde-frameworks-devel, Konversation

Maniphest Tasks: T11577

Differential Revision: https://phabricator.kde.org/D24597