* saml auth: `authnContext` must be a list now

This fixes

    this.options.authnContext.forEach is not a function

when trying to login via SAML on wiki-js 2.5.281.

Reason for that is that `authnContext` must be a list now which is
apparently a breaking change that was missed while upgrading
passport-saml[1].

Resolves #5289

[1] https://github.com/node-saml/passport-saml/pull/615

* fix(auth): split authnContext for SAML authenticaiton module
Co-authored-by: Nicolas Giard <github@ngpixel.com>