tests: make upgrade tests more robust

[czeslavo]

What this PR does / why we need it:

It introduces two main changes to the upgrade tests that should make them more reliable:

1. Ensure that after the controller deployment is upgraded, we actually wait until all new replicas are rolled out and become ready so that we're sure that we're testing against the new version.
2. Add a new path to the Ingress after the upgrade is done and test against it to make sure that the new configuration is processed correctly (without that we were only verifying that the old Kong Gateway instance was configured, but it could be the configuration pushed by the old controller).

https:/Kong/kubernetes-ingress-controller/actions/runs/5142252037/jobs/9255887769 test failure proves that if we set the feature gate FillIDs=true, the upgrade will fail. To fix that we will bump deck version to v1.21.0 which will make the test pass.

Which issue this PR fixes:

Noticed when working on #4025.

[team-k8s-bot]

E2E (targeted) tests were started at https:/Kong/kubernetes-ingress-controller/actions/runs/5135464942

[pmalek]

Some nits for consideration