[REQUEST]: Rename "Elastic instance" to "Elastic Cloud resource"

### Description

renaming the generic "instances" umbrella term for ECH deployments + serverless projects to "Elastic Cloud resources"

excludes instances that comprise a deployment

todo:
* [x] update narrative docs in docs-content https://github.com/elastic/docs-content/pull/1136
* [x] check cloud tooling docs in cloud (done: no instances found)
* [x] check API docs
* [ ] update API doc strings

### Resources

see https://github.com/elastic/cloud/issues/139405

### Which documentation set does this change impact?

Elastic Cloud (Hosted and Serverless) only

### What release is this request related to?

N/A

### Collaboration model

The documentation team

### Point of contact.

**Main contact:** @alxchalkias

## Action
added by (@georgewallace)
- Update the cloud api doc strings