Items expects a single field, not a list (elastic#233414)

## Summary

From the
[spec](https://swagger.io/docs/specification/v3_0/data-models/data-types/#arrays)
the `items` field should be a single schema defining the types within an
array. If the array may contain multiple types then the schema should
combine those types via `oneOf`.

Related to elastic#228077


### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

- [x] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)
- [x]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [x] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [x] 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.
- [x] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)
- [x] Review the [backport
guidelines](https://docs.google.com/document/d/1VyN5k91e5OVumlc0Gb9RPa3h1ewuPE705nRtioPiTvY/edit?usp=sharing)
and apply applicable `backport:*` labels.

---------

Co-authored-by: kibanamachine <[email protected]>

Author: 2 people

oas_docs/output/kibana.yaml

@@ -63078,7 +63078,7 @@ paths:
                     ]
               schema:
                 items:
-                  - $ref: '#/components/schemas/Synthetics_getParameterResponse'
+                  $ref: '#/components/schemas/Synthetics_getParameterResponse'
                 type: array
           description: A successful response.
       summary: Get parameters

src/platform/packages/private/kbn-validate-oas/oas_error_baseline.json

@@ -1,4 +1,4 @@
 {
-  "./oas_docs/output/kibana.yaml": 314,
+  "./oas_docs/output/kibana.yaml": 312,
   "./oas_docs/output/kibana.serverless.yaml": 294
 }

x-pack/solutions/observability/plugins/synthetics/docs/openapi/synthetic_apis.yaml

@@ -522,7 +522,7 @@ paths:
               schema:
                 type: array
                 items:
-                  - $ref: "#/components/schemas/getParameterResponse"
+                  $ref: "#/components/schemas/getParameterResponse"
               examples:
                 getParametersResponseExample1:
                   summary: Read access