Change deprecated preferred & required to selectors in Player Capacity docs

[JJhuk]

What type of PR is this?
/kind documentation

What this PR does / Why we need it:
Because preferred and required fields are deprecated in agones, so I changed them to selectors

Which issue(s) this PR fixes:

Closes #2570

[agones-bot]

Build Failed 😱

Build Id: be7b8556-8d6c-4c5d-860f-4a482015f8dd

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[agones-bot]

Build Succeeded 👏

Build Id: e94b38be-c034-46f9-9d08-ed4db6ac1073

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.24.0-464f74d-amd64
• image: gcr.io/agones-images/agones-ping:1.24.0-464f74d
• Linux C++ SDK (build): agonessdk-1.24.0-464f74d-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.24.0-464f74d.zip

A preview of the website (the last 30 builds are retained):

• https://464f74d-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2577/head:pr_2577 && git checkout pr_2577
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.24.0-464f74d-amd64

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JJhuk, roberthbailey

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