[NXCM-5131] Change item age value type from Integer to int

The problem is that NuGet is not configuring these values at all. The default value for an int while serializing is `0`, default value for Integer is `null` which explains the different values for the age configuration in the UI. (0<>'empty value').

https://bamboo.zion.sonatype.com/browse/NXO-OSSF14-1