Fix MantisCore plugin dependencies

In issue 0017360, we introduced setting a default maximum version for
MantisCore to avoid compatibility issues by disabling plugins which have
not been updated for a new Mantis release, with a break on every minor
version (x.Y.z).

With 2.0 and the decision to only introduce breaking changes on major
versions (X.y.z), this code has to be changed.

Fixes 0022171