Prepare migration to IsovalentEgressGatewayPolicy resources

[simu]

We ensure that CiliumEgressGatewayPolicy resources aren't pruned automatically, so that we'll be able to migrate to IsovalentEgressGatewayPolicy resources without interruptions of egress traffic.

Checklist

• The PR has a meaningful title. It will be used to auto-generate the
  changelog.
  The PR has a meaningful description that sums up the change. It will be
  linked in the changelog.
• PR contains a single logical change (to build a better changelog).
• Categorize the PR by adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog.

[simu]

Initial testing didn't show any negative impacts of having two copies of the same EgressGatewayPolicy in place at the same time. Probably still makes sense to do the migration in a planned maintenance window.