[OpenAPI] Fix Serverless API base URL

[lcawl]

Summary

Fixes elastic/docs-content#153

In particular, it removes unnecessary server URL declarations from some SLO API files and updates the kibana API overlays so that the serverless API document is no longer referring to "localhost".

NOTE: Per https://spec.openapis.org/oas/v3.0.3#server-object the "default" value is required.

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines

Identify risks

Does this PR introduce any risks? For example, consider risks like hard to test bugs, performance regression, potential of data loss.

No

Screenshots

The Kibana Serverless API examples will look like this:

The Kibana API examples will look like this:

[elasticmachine]

Pinging @elastic/kibana-docs (Team:Docs)

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 756fb85

Metrics [docs]

✅ unchanged