Update example operator.yaml to support vtgate HPA

[siadat]

Description

Per planetscale/vitess-operator#598 (comment), this PR ensures examples/operator/operator.yaml is in sync with the operator, when that PR is merged.

Related Issue(s)

planetscale/vitess-operator#598

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

This PR contains the changes need to auto-scale vtgate deployments by the operator.

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
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Documentation

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Backward compatibility

• Protobuf changes should be wire-compatible.
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• vtctl command output order should be stable and awk-able.