Validation to prevent deletion of actively referenced Revisions

/area API

Intended as follow up to: https://github.com/knative/serving/issues/8208

## Describe the feature

We will start labelling revisions with pending/active/retired states such that all in-progress deployments and actively serving revisions should be marked. We also recently added DELETE support to the webhooks for validation.

We should
* Create a feature flag for prevent-actively-serving-delete validation
* Create a new hook that fails validation when pending/active are present on a revision
* e2e test trying to delete an active revision
* e2e test that cascading deletions are not blocked

This feature can remain optional, but it is likely that users don't intend to delete revisions that are active/pending. This will make it important to ensure pending states are resolved within the pending deployment time. We will want to give users a nice error message about why their deletion request has failed and may want to consider a --force option to bypass validation (actually we could consider such a header generically for bypassing validation, maybe that's a separate issue).

Start with the existing DELETE validation support in the webhooks and the revision pending/active/retired states described here, then review follow-up issue #8208 for context. Define the feature flag and validation behavior, add an end-to-end test for deleting an active revision, and verify that cascading deletions remain allowed.

Written by the indexing model from the issue text.