make the libssh finder ensure the new ssh target is set

Summary:
starting with 0.9.2 the libssh cmake config defines a new ssh imported
target, use this as the new gold standard and ensure older versions
are compatible

broken ubuntu: no cmake config -> manual finder -> target injected
0.9.0 and earlier: cmake config -> target injected
0.9.2: cmake config -> target already defined; noop

(0.9.1 is broken as it neither matches the old nor the new expectation)

Test Plan:
no cmake config -> target injected
older cmake config -> target injected
newer cmake config (with merge request) -> noop

Subscribers: kde-frameworks-devel, kfm-devel

Tags: Dolphin, Frameworks

Differential Revision: https://phabricator.kde.org/D25170