Update branch in logstash-api.yaml

[lcawl]

Release notes

[rn:skip]

What does this PR do?

This PR updates the branch number in the introduction of the Logstash OpenAPI document.
When 9.2 is released, it will affect the information that appears in https://www.elastic.co/docs/api/doc/logstash/v9/#topic-documentation-source-and-versions

Why is it important/What is the impact to the user?

This section of the API documentation explains where to find the source files.

Checklist

- [ ] My code follows the style guidelines of this project
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
- [ ] I have added tests that prove my fix is effective or that my feature works

Author's Checklist

N/A

How to test this PR locally

A preview can be created by installing the Bump.sh CLI and running the preview command.

Related issues

N/A

Use cases

N/A

Screenshots

TBD

Logs

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