[pop3 kioslave] Port KTcpSocket (deprecated) to QSslSocket

Summary:
This change is mainly to make pop3 kioslave ready for an upcoming change
in KIO (possibly 5.65.0) TcpSlaveBase which will use QSslSocket instead
of KTcpSocket (the latter is being deprecated).

Reviewers: mlaurent, vkrause

Reviewed By: vkrause

Subscribers: vkrause, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D24929