Fix swagger schema error after #15946

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Following the merge of #15946 the Swagger schema generation reports this error:

An unhandled exception has occurred while executing the request.
Swashbuckle.AspNetCore.SwaggerGen.SwaggerGeneratorException: Failed to generate Operation for action - Umbraco.Cms.Api.Management.Controllers.DocumentVersion.ByKeyDocumentVersionController.ByKey (Umbraco.Cms.Api.Management). See inner exception
 ---> Swashbuckle.AspNetCore.SwaggerGen.SwaggerGeneratorException: Failed to generate schema for type - Umbraco.Cms.Api.Management.ViewModels.Document.DocumentVersionResponseModel. See inner exception
 ---> System.InvalidOperationException: Can't use schemaId "$ContentForDocumentResponseModel" for type "$Umbraco.Cms.Api.Management.ViewModels.Content.ContentResponseModelBase`2[Umbraco.Cms.Api.Management.ViewModels.Document.DocumentValueModel,Umbraco.Cms.Api.Management.View
Models.Document.DocumentVariantResponseModel]". The same schemaId is already used for type "$Umbraco.Cms.Api.Management.ViewModels.Document.DocumentResponseModelBase`2[Umbraco.Cms.Api.Management.ViewModels.Document.DocumentValueModel,Umbraco.Cms.Api.Management.ViewModels.Document.DocumentVariantResponseModel]"

This PR fixes it.

Testing this PR

1. Verify that the Swagger doc can load.
2. Test that the document versioning endpoints from Document version endpoints #15946 still work.