Merge pull request #55321 from xiangpengzhao/remove-1.5thing

Automatic merge from submit-queue (batch tested with PRs 53780, 55663, 55321, 52421, 55659). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Remove the comparison of ReadyReplicas to zero.

**What this PR does / why we need it**:

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
ref: #43465

**Special notes for your reviewer**:
AFAIK, we have already stopped supporting upgrades from 1.5.
cc @fejta @krzyzacy 

**Release note**:

```release-note
NONE
```

pkg/apis/extensions/validation/validation.go

@@ -325,9 +325,7 @@ func ValidateDeploymentStatus(status *extensions.DeploymentStatus, fldPath *fiel
 	if status.AvailableReplicas > status.Replicas {
 		allErrs = append(allErrs, field.Invalid(fldPath.Child("availableReplicas"), status.AvailableReplicas, msg))
 	}
-	// TODO: ReadyReplicas is introduced in 1.6 and this check breaks the Deployment controller when pre-1.6 clusters get upgraded.
-	// 		 Remove the comparison to zero once we stop supporting upgrades from 1.5.
-	if status.ReadyReplicas > 0 && status.AvailableReplicas > status.ReadyReplicas {
+	if status.AvailableReplicas > status.ReadyReplicas {
 		allErrs = append(allErrs, field.Invalid(fldPath.Child("availableReplicas"), status.AvailableReplicas, "cannot be greater than readyReplicas"))
 	}
 	return allErrs