Remove outdated Tasklist CSRF documentation

[npepinpe]

Description

This PR removes the Tasklist CSRF documentation, as it's now superseded by the Orchestration Cluster's CSRF docs in general.

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 Documentation 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

• The commit history of this PR is cleaned up, using {type}(scope): {description} commit message(s)

• My changes are for an upcoming minor release and are in the /docs directory (version 8.9).
• My changes are for an already released minor and are in a /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.

[Copilot]

Pull Request Overview

This PR removes the outdated Tasklist-specific CSRF protection and clustering documentation sections, which have been superseded by the Orchestration Cluster's core settings documentation.

• Removes Tasklist-specific CSRF protection documentation (approximately 27 lines)
• Removes Tasklist clustering and distributed user sessions documentation (approximately 15 lines)
• Applies changes to both version 8.8 and 8.9 (docs/) documentation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/self-managed/components/orchestration-cluster/tasklist/tasklist-configuration.md	Removes outdated CSRF and clustering sections from the 8.9 Tasklist configuration documentation
versioned_docs/version-8.8/self-managed/components/orchestration-cluster/tasklist/tasklist-configuration.md	Removes outdated CSRF and clustering sections from the 8.8 Tasklist configuration documentation

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[mesellings]

Lgtm 🚀