Use `VetMassKillCredit` over `VetExperience` in the UI when a unit reached maximum veterancy

### Description

The field `VetMassKillCredit` is introduced by https://github.com/FAForever/fa/pull/6110 but it is not used yet. As a result the value will not show up to the user.

### Course of Action

Extend [unitview.lua](https://github.com/FAForever/fa/blob/develop/lua/ui/game/unitview.lua) to retrieve and use the unit statistic `VetMassKillCredit` when a unit has a veterancy level of 5 or higher. This should be a relative small change where `VetLevel` and `VetExperience` are already retrieved. When the value of `VetLevel` is 5 or more, instead of retrieving `VetExperience` you retrieve `VetMassKillCredit`.

### Test Plan

Verify that the correct value is used when a unit switches from veterancy 4 to veterancy 5.