Replace Subject with login_hint

[blombard]

Context

We should not use Subject anymore but use login_hint instead.

Perhaps we could also remove all code related to name_identifier_value_requested in this PR or a follow-up one? (Or maybe not if some IdPs are still using it?)

Microsoft doc

https://learn.microsoft.com/en-us/entra/identity-platform/single-sign-on-saml-protocol#subject

Don't include a Subject element. Microsoft Entra ID doesn't support specifying a subject in AuthnRequest and will return an error if one is provided.

A subject can instead be provided by adding a login_hint parameter to the HTTP request to the single sign-on URL, with the subject's NameID as the parameter value.

Google doc

https://support.google.com/a/answer/15544042?hl=en#

Note: Google's SAML IdP doesn't use the NameID that may be present in the Subject element of the AuthnRequest. Use login_hint instead.

Error

[pitbulk]

@blombard can you provide a PR where we add to the documentation the use of the login_hint trick rather than remove the use of Subject.

The fact that Google/MSFT accept this parameter does not mean that others accept it.
If I'm not wrong, login_hint is not part of the SAML standard itself.