Consider removing one level of the OIDC Backchannel Logout DSL

[jzheaux]

To active OIDC Back-Channel Logout support in the DSL, an application does this:

http
    .oidcLogout((oidc) -> oidc.backChannel(Customizer.withDefaults())
    )

This could be simplified to:

http
   .oidcBackChannelLogout(Customizer.withDefaults())

This would be place the logout DSL at the same level as other logout DSLs:

http
    .logout((logout) -> logout ...)
    .saml2Logout((saml2) -> saml2 ...
    .oidcBackChannelLogout((oidc) -> oidc ...)

Also, it's less nesting which often makes the DSL more navigable.

This would mean deprecating the existing backChannel DSL method with the intent to remove in the next major version.