[DOCS][OAS] Add cases to Kibana API bundle

[lcawl]

Summary

This PR:

• reverts [OAS] Migrate case APIs to v3.1.0  #171593 so that the cases openAPI document can be joined with the other documents in oas_docs
• cleans up the operation summaries as suggested by the linting rules
• adds the deprecated property per [HTTP/OAS] Support deprecated field #181240
• removes the /s/ version of the paths from the openAPI document, since per discussions elsewhere we'll cover that in an introductory page rather than repeat for every endpoint

[obltmachine]

🤖 GitHub comments

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)

[kibana-ci]

💚 Build Succeeded

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• Commit: a3ea497

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[karenzone]

LGTM

[kibanamachine]

💔 All backports failed

Status	Branch	Result
❌	8.15	Backport failed because of merge conflicts

Manual backport

To create the backport manually run:

node scripts/backport --pr 189020

Questions ?

Please refer to the Backport tool documentation

[lcawl]

💚 All backports created successfully

Status	Branch	Result
✅	8.15

Note: Successful backport PRs will be merged automatically after passing CI.

Questions ?

Please refer to the Backport tool documentation