QSslSocket: add TLS 1.1 and TLS 1.2 support.
Backport 'Support for DH and ECDH key exchange for QSslSocket servers' from Qt 5.

See https://codereview.qt-project.org/#/c/82817/ for more info.

Qt 5 qtbase commit: 814a1c7b2b940cdf6b1716406b26063576ac0d95