ItemCategory exposes public static MIN_TOSHOW field to API

In https://github.com/jenkinsci/jenkins/commit/25f9c99d8376667b2efaabe8cb6f5b4be6024618 recena introduced a new "public static int MIN_TOSHOW = 1" field. So we have a constant, which can be occasionally modified by any API user.

This field is now a part of API, so we cannot easily convert it to final or make private. My proposal is to restrict it at least