Deprecate/remove relaxed media types

Recent versions of kiota have already been fixed for the appearance of double quotes in media types. NSwag was fixed in https://github.com/RicoSuter/NSwag/pull/5345. Once NSwag has released a new version, the "relaxed" workaround is no longer needed. For compatibility, the .NET properties should be marked with `[Obsolete]`, but the relaxed forms should be removed from the OpenAPI JSON:API extension spec.