OpenAPI deployment warning

Relates to https://github.com/elastic/logstash/pull/17292

#### Tell us about the issue

**Description:**

When the latest OpenAPI document was uploaded from https://github.com/elastic/logstash/blob/main/docs/static/spec/openapi/logstash-api.yaml
to https://www.elastic.co/docs/api/doc/logstash/, the following message appeared in the deployment log:

> AllOf keyword can only be used in a Schema Object. It has been ignored while trying to parse a SchemaProperty Object.

This is not a message we saw when we linted the document.
My assumption from a quick look at the OpenAPI document is that it's likely related to items like this:

```
components:
schemas:
...
worker_millis_per_event:
description: |
This metric is expressed in ...
allOf: # need to use allOf to keep the description
- $ref: "#/components/schemas/FlowWindows"
```

**URL:**

https://www.elastic.co/docs/api/doc/logstash/

**Anything else?**

I think the investigation would need to be related to (1) how this appears in the published docs now (2) whether there are other supported methods to accomplish what is being done here, and (3) if not, whether this is something that *is* supported by OpenAPI and *ought* to be supported by Bump.sh

Start with docs/static/spec/openapi/logstash-api.yaml and compare the published result at elastic.co/docs/api/doc/logstash/ with the deployment warning and existing lint output. Investigate the worker_millis_per_event allOf usage, determine whether a supported OpenAPI alternative exists, and document whether the warning reflects a real publication issue or Bump.sh support gap.

Written by the indexing model from the issue text.