docs(server): full copy-edit of auth mode page

[agilgur5]

I linked to this in Slack recently and realized how dated it was 😬
Also aligns with my other docs copy-edit PRs

Motivation

• this page had some out-of-date information, had inconsistent terminology, and lacked links to several of the features it mentioned
  • it sorely needed an update

Modifications

• remove outdated statement about RBAC mapping

  • this feature has existed since SSO RBAC was implemented (feat(server): Enable RBAC for SSO. Closes #3525 #4198), so replace with a link to that feature
• terminology & consistency improvements:

  • "choose" -> "configure"
  • "which kube config" -> how to "authenticate"
    • there is no kube config in some of these cases either, so this terminology was not only inconsistent with the rest of the docs (and itself), but also very confusing
    • also that an SA generates a kube config is a k8s implementation detail that not all users are aware of. stick to a higher level description
  • consistently capitalize "Server"
  • use "v{version} and after" consistently, per docs: consistently use "> v{version} and after" #12581
    • also mention the current default first, not the old default
      • the current has been around for years and is more important than the old one, so it should be first
    • remove the redundant SSO version references here as they are already listed in more consistent style within the linked pages
  • use code backticks consistently when referring to one of the modes and when referring to the CLI flag

• add links to mentioned features:

  • to hosted mode vs. local mode
  • to k8s bearer token docs
  • to SSO and NS delegation

Verification

make docs passes

Future Work

This page is pretty tiny, it might make sense to merge with the general Argo Server page, which links to this page, which then links back to it 😕