[OpenAPI3] Multiple responses with same status code should use `anyOf` instead of `oneOf`



[Playground Link](https://cadlplayground.z22.web.core.windows.net/?c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7Cgp1c2luZyBUeXBlU3BlYy5IdHRwOwpAc2VydmljZSh7CiAgdGl0bGU6ICJXaWRnZXQgU8YbIiwKICB2ZXJzaW9uOiAiMS4wLjAiLAp9KQpuYW1lc3BhY2UgRGVtb8cvOwoKb3AgdGVzdDEoKTogxFhAc3RhdHVzQ29kZSBfOiAyMDA7CiAgxD46IHN0cmluZzsKfSB83yzLLDsK&e=%40typespec%2Fopenapi3&options=%7B%7D)