[CMAKE] Improve FindMiniUPnPc

Summary:
This creates an imported target on the model of D5339 and sets the
minimum version to 1.5.
Older versions might be supported as well but net.cpp has code to
handle 1.5 at least, and it is old enough (2011).

Test Plan:

ninja check

Run the Gitian builds.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5450