Automatic merge from submit-queue (batch tested with PRs 51471, 50561, 50435, 51473, 51436)

Feature gate initializers field

The metadata.initializers field should be feature gated and disabled by default while in alpha, especially since enforcement of initializer permission that keeps users from submitting objects with their own initializers specified is done via an admission plugin most clusters do not enable yet.

Not gating the field and tests caused tests added in https://github.com/kubernetes/kubernetes/issues/51429 to fail on clusters that don't enable the admission plugin.

This PR:
* adds an `Initializers` feature gate, auto-enables the feature gate if the admission plugin is enabled
* clears the `metadata.initializers` field of objects on create/update if the feature gate is not set
* marks the e2e tests as feature-dependent (will follow up with PR to test-infra to enable the feature and opt in for GCE e2e tests)

```release-note
Use of the alpha initializers feature now requires enabling the `Initializers` feature gate. This feature gate is auto-enabled if the `Initialzers` admission plugin is enabled.
```