tst_qwebsocketserver: force TLS 1.2 for the PSK test

When using TLS 1.3 the psk callback is called on the start of a
connection. Force TLS 1.2 to avoid this.

Task-number: QTBUG-71560
Change-Id: I97df3a9d2f100d10b4a1d2e8d0e4696688ec66ac
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
(cherry picked from commit 7022a1f7)