Update operator documentation and API spec for `.spec.sso.dex`

Introduction of `.spec.sso.dex` requires updating the argocd-operator documentation to outline this new method of configuring dex, along with the deprecation warning for `.spec.dex`. The API Spec should also be updated to reflect the appropriate content 

Acceptance criteria

• Usage guide is updated with new instructions

• API spec is updated with new structure
• Update GitOps operator User guide 
• Add release-note-text to apt Jira story