refactor(core): Simplify state in test webhooks (no-changelog) (#8155)

This PR simplifies state in test webhooks so that it can be cached
easily. Caching this state will allow us to start using Redis for manual
webhooks, to support manual webhooks to work in multi-main setup.

- [x] Convert `workflowWebhooks` to a getter - no need to optimize for
deactivation
- [x] Remove array from value in `TestWebhooks.webhookUrls`
- [x] Consolidate `webhookUrls` and `registeredWebhooks`

packages/cli/src/AbstractServer.ts

@@ -214,7 +214,7 @@ export abstract class AbstractServer {
 			// TODO UM: check if this needs validation with user management.
 			this.app.delete(
 				`/${this.restEndpoint}/test-webhook/:id`,
-				send(async (req) => testWebhooks.cancelTestWebhook(req.params.id)),
+				send(async (req) => testWebhooks.cancelWebhook(req.params.id)),
 			);
 		}