Relax foreman proxy compatibility check condition
Based on discussion: https://community.theforeman.org/t/core-proxy-version-comparison/10600
Correct condition definition: foreman proxy X.Y version should be at least as Foreman core's version, but no more than one Y version ahead of it.
foreman <-> proxy:
1.18.1 <-> 1.18.0 : OK
1.19.0 <-> 1.18.0 : Warn
1.18.0 <-> 1.19.0 : OK
1.18.0 <-> 1.20.0 : Warn