fix: don't flap DataPlane ready status on rolling update

[pmalek]

What this PR does / why we need it:

This changes how DataPlane defines its Ready status condition.

Previously it has been:

if status.AvailableReplicas < status.Replicas {
  // not ready
}

This PR proposes the following:

if status.AvailableReplicas < spec.Replicas {
  // not ready
}

This prevents DataPlane flapping its Ready status condition during a rolling update

Which issue this PR fixes

Fixes #

Special notes for your reviewer:

Related: https://github.com/Kong/gateway-operator-archive/pull/1348/

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect significant changes