[7.7] Document new auth provider configuration options and login selector

[legrego]

#53010 introduced new kibana.yml settings for configuring authentication providers. Specifically, there is a new configuration format, and there is support for multiple providers of the same type.

Additionally, there is an optional Login Selector UI which can be configured with 1 or more of the configured providers.

Our documentation should be updated to reflect these new options, and the breaking change docs for 8.0 should note that the old config settings won't be supported.

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[azasypkin]

Note to myself: also remove mentioning of server.xsrf.whitelist setting in our SAML/OIDC docs in this scope of this issue.