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

[Garanas]

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 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.