OpenAPI schema for forms uses allOf incorrectly

Already asserted here:

https://github.com/dotnet/aspnetcore/blob/4061277cd4bc0c8dbcca300f46decada040ebc7c/src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_2/OpenApiDocumentIntegrationTests.VerifyOpenApiDocument_documentName%3Dforms.verified.txt#L82-L98

This use of allOf sounds wrong.

allOf doesn't mean "I want both properties to exist". It means I want my schema to validate against all the child schemas under allOf.

In addition, the parent schema defined two required properties, `resume` and `files`, but The parent schema doesn't define any property at all. The properties are defined in child schemas under allOf.

See also https://spec.openapis.org/oas/v3.2.0.html#example-basic-multipart-form