add xor to oidc configuration

[hisImminence]

Description

This PR is related to the comment in the this ticket: #4619 (comment).

Tltr: Its adds an or statement to the oidc configuration as it was not clear to users it both, envVars and/or helm values need to be set. Changes to back to 8.4 guides (no guide for OIDC in 8.3).

However, this ticket is not closed as another question related to oidc came up.

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support. (add bug or support label)
• This is already available but undocumented and should be released within a week. (add available & undocumented label)
• This is on a specific schedule and the assignee will coordinate a release with the DevEx team. (create draft PR and/or add hold label)
• This is part of a scheduled alpha or minor. (add alpha or minor label)
• There is no urgency with this change (add low prio label)

PR Checklist

• My changes are for the next minor and are in /docs directory (aka /next/).
• My changes are for an already released minor and are in /versioned_docs directory.

• I added a DRI, team, or delegate as a reviewer for technical accuracy and grammar/style:
  • Engineering team review
  • Technical writer review via @camunda/tech-writers unless working with an embedded writer.