fix: give correct unauthorized response on OpenID Connect Auth

[rhuanbarreto]

According to https://www.rfc-editor.org/rfc/rfc9110.html#section-15.5.2 the correct response in this case of missing value is 401.

[github-actions]

📝 Docs preview for commit 854bba3 at: https://641ef02df7a788363926a64f--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit 379c716 at: https://641f0bfa8d28a94769db5ac3--fastapi.netlify.app

[tiangolo]

Thanks @rhuanbarreto, but with a 401:

MUST send a WWW-Authenticate header field

And as it's not clear which WWW-Authenticate header field should be used, not providing it is actually invalid, and although 403 is not the best error code, it's a valid option. So, for now, I'll leave it as is.

Later I'll do some more research to see if there's a simple way to provide the correct WWW-Authenticate values and then allow using 401.

But thanks for the interest! ☕