Base readiness of EPP on availability of its NATS-connection

[friedrichwilken]

Description

EPP should not report itself as ready when it is not able to send events to NATS. In an upgrade situation this should prevent a short downtime when the EPP currently reports as ready, but rejects events as it is not connected to NATS yet.

In this issue we decoupled epp readiness from nats-connection availablility. kyma-project/kyma#18298

this is a temporary fix. as soon as eventing manager was release, let's revert this.

During upgrade testing we noticed a short period

Acceptance

• run load test to verify that during an upgrade no events are rejected by a non ready EPP
  -> this change did not fix, that no events are rejected. This is to be solved in EPP Image Update leads to NACKS #52
• Documentation is updated (on-call manual): documentation/#463
• SRE is notified about the change

Hint

• depends on Setup pipelines for EPP #45