OpenAPI Check Workflow (Annotations) not working correctly

The [OpenAPI check](https://github.com/bundesAPI/api-doc-template/blob/main/.github/workflows/openapi_check.yaml) is currently not working correctly.

The annotations which show warnings or errors for the openapi file are sometimes not added at all or sometimes just added to another workflow run.

See: https://github.com/stoplightio/spectral-action/issues/646

Maybe we have to fix the action by ourself since it looks like stoplight does not work on the github action anymore.

We should also think about testing both (or all) openapi files if they exist.

Start with .github/workflows/openapi_check.yaml and compare its behavior with the linked stoplightio/spectral-action issue #646. Confirm how annotations are attached to workflow runs, then verify that warnings and errors appear on the correct run and that both or all OpenAPI files are checked when present.

Scritto dal modello di indicizzazione a partire dal testo della issue.