Subscription hashes for EventMesh webhooks are reset if the secret is changed

[marcobebway]

Description

In case the EventMesh is the active backend, the precomputed hashes of the Kyma subscriptions are reset if the secret is changed. This results in deleting and recreating the EventMesh webhooks.

Steps to reproduce

1. Provision a Kyma cluster
2. Apply an eventing-webhook-auth secret
3. Switch backend to EventMesh
4. Set EVENTING_WEBHOOK_AUTH_ENABLED=true
5. Create a Kyma subscription
6. Take a note of the creation timestamp of the EventMesh subscription
7. Apply another eventing-webhook-auth secret
8. Take a note of the creation time stamp of the EventMesh subscription
9. Both creation timestamps don't match, which means the EventMesh subscription was deleted and recreated

Acceptance criteria

• The precomputed hashes are preserved.
• The EventMesh webhooks are not deleted if the secret changed.