[Internal]: Self-managed Stack Upgrade should include air-gapped components

### Description

When working in Air-gapped environment, I noticed that our public doc on self-managed stack upgrade is insufficient. It should include additional air-gapped components like [EPR](https://www.elastic.co/docs/reference/fleet/air-gapped#air-gapped-diy-epr), [EAR](https://www.elastic.co/docs/reference/fleet/air-gapped#host-artifact-registry), Elastic Endpoint Artifact Repo and Elastic Maps into the list.

Here are the list of docs mentioning the stack components:
- https://www.elastic.co/docs/deploy-manage/upgrade/plan-upgrade#conduct-a-component-inventory
- https://www.elastic.co/docs/deploy-manage/upgrade/plan-upgrade#upgrade-order
- https://www.elastic.co/docs/deploy-manage/upgrade/deployment-or-cluster/self-managed

### Resources

https://www.elastic.co/docs/deploy-manage/deploy/self-managed/air-gapped-install

### Which documentation set does this change impact?

Elastic On-Prem only

### Feature differences

N/A

### What release is this request related to?

N/A

### Serverless release

N/A

### Collaboration model

The documentation team

### Point of contact.

**Main contact:** N/A

**Stakeholders:** N/A