[OAS] Refresh saved objects API document

[lcawl]

Summary

This API updates the manually-maintained openAPI document for saved objects as follows:

1. Reverts from OAS 3.1 to 3.0, so that it can be joined with other 3.0 files and aligns with the version of the automated document. This involved updating the way an example was declared.
2. Adds a serverless-specific list of endpoints, since it's a smaller subset per comments in [HTTP/OAS] Add tags and summaries for saved object APIs #184184
3. Edits the operation summaries and descriptions to align with linting rules

[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!)

[kilfoyle]

LGTM! 🦕

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 4253520

Failed CI Steps

• FTR Configs #99

Metrics [docs]

✅ unchanged

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