Updating NGSI-LD OAS v1.8.1 operations already considered in v1.7.1: 1)...

Updating NGSI-LD OAS v1.8.1 operations already considered in v1.7.1: 1) Including new query parameters, some descriptions, and headers for GET operations within the following endpoints: /entities, /entities/{entityId}, /temporal/entities, and /temporal/entities/{entityId}; 2) Updating and including the definition of the new/updated query parameters within the Parameters section of the NGSI-LD OAS; 3) Including error code 504 GatewayTimeout instead of 503 ServiceUnavailable for JSON-LD Context Access Points within the DELETE /jsonldContext/{contextId} operation; 4) Including GeoJSON FeatureCollection as return data type within the HTTP 200 response body of POST /entityOperations/query operation.