Improvements and fixes for the Item schema

[m-mohr]

Related Issue(s): None

Proposed Changes:

1. It's not a bug, but the oneOf is not needed in the schema. Therefore removed it for simplicity.
2. Removed the "geometry" and "type" as required property as it is already required in the GeoJSON Feature schema (and we do not redefine it in the schema, therefore it is referring to a non-existing property anyway).
3. Added "bbox" as property so that the "require" flag really works.
4. Fixed a typo: "updated"->"created"

PR Checklist:

• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• API only: I have run npm run generate-all to update the generated OpenAPI files.