Update example allocation yaml files to use selectors instead of required

[roberthbailey]

Is your feature request related to a problem? Please describe.
examples/gameserverallocation.yaml uses the newer selectors but a lot of the yaml files under the /examples directory still use required (e.g. examples/simple-game-server/gameserverallocation.yaml and examples/xonotic/gameserverallocation.yaml but probably others as well).

Describe the solution you'd like
Search through the game server allocation yaml files checked into the repository and change the spec to use selectors instead of required

Describe alternatives you've considered
n/a

[markmandel]

I have the updates for STK example in a branch, I'd just forgotten about it. I'll publish this one.