fix: ensure that Gateway managed DataPlanes are reduced

[pmalek]

Problem statement

It seems that there was one step that was missed in reduction of resources, namely Gateway's managed DataPlanes.

When for some reason there was more than 1 DataPlane owned by 1 Gateway at any point in time KGO never removed those:

gateway-operator/controllers/gateway/controller.go

Lines 384 to 391 in 2255cec

	if count > 1 {
	err = fmt.Errorf("data plane deployments found: %d, expected: 1", count)
	k8sutils.SetCondition(
	createDataPlaneCondition(metav1.ConditionFalse, k8sutils.UnableToProvisionReason, err.Error(), gateway.Generation),
	gatewayConditionsAndListenersAware(gateway),
	)
	return nil, err
	}

.

This PR introduced the DataPlane reduction step which will remove the newest DataPlane which exists when there's more than 1 of those per Gateway.

Which issue does this PR solve

Fixes #29

---

This change has also the following effect on integration tests runtime:

Before: https://github.com/Kong/gateway-operator/actions/runs/8359101231

After: https://github.com/Kong/gateway-operator/actions/runs/8362324992?pr=43

[pmalek]

We might want to look into retrying the installation of Gateway API CRDs as it seems it can take the whole pipeline down:

panic: ERROR: command "/usr/bin/kubectl --kubeconfig /tmp/-kubeconfig-1b8f9f74-eef2-461b-bfa9-57644bbfe25b146573200 -v9 apply -k github.com/kubernetes-sigs/gateway-api/config/crd/experimental?ref=v1.0.0" failed STDERR=() STDERR=(I0320 17:25:09.195247   21852 loader.go:395] Config loaded from file:  /tmp/-kubeconfig-1b8f9f74-eef2-461b-bfa9-57644bbfe25b146573200
error: failed to run '/usr/bin/git fetch --depth=1 https://github.com/kubernetes-sigs/gateway-api v1.0.0': fatal: unable to access 'https://github.com/kubernetes-sigs/gateway-api/': HTTP/2 stream 1 was not closed cleanly before end of the underlying stream
: exit status 128
): exit status 1


goroutine 1 [running]:
github.com/kong/gateway-operator/test/integration.exitOnErr({0x563eea0, 0xc000c72060})
	/home/runner/work/gateway-operator/gateway-operator/test/integration/suite.go:203 +0x14d
github.com/kong/gateway-operator/test/integration.TestMain(0xc000[85](https://github.com/Kong/gateway-operator/actions/runs/8363207891/job/22895613453?pr=43#step:4:86)45a0, 0xc0007d5340)
	/home/runner/work/gateway-operator/gateway-operator/test/integration/suite.go:157 +0x1325
github.com/kong/gateway-operator/test/integration_test.TestMain(0xc0008545a0)
	/home/runner/work/gateway-operator/gateway-operator/test/integration/run_integration_test.go:28 +0x216
main.main()
	_testmain.go:85 +0x2f5