Since 7.3, it's possible to explicitly configure the SAML realm to
be used in Kibana's configuration. This in turn, eliminates the need
of properly setting `xpack.security.public.*` settings in Kibana
and largely simplifies relevant documentation.
This also changes `xpack.security.authProviders` to
`xpack.security.authc.providers` as the former was deprecated in
favor of the latter in 7.3 in Kibana
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