archive(8.1) exclude: remove versions 8.1 and 3.9.0, part 1: reviewable steps

[pepopowitz]

BLOCKED

• The unsupported/8.1 site is not yet in production. It was released in https://github.com/camunda/camunda-docs/releases/tag/8.1.37!

Description

Part of #3731.

Includes the smaller items for removing version 8.1 from the main docs, to keep the PR in a reviewable state.

Included in this PR

• Remove v8.1 & Optimize v3.9.0 from the version manifest files, so that they're not included in the deployed site
• Redirect all 8.1 / 3.9.0 traffic to unsupported.docs.camunda.io/8.1 (or stage.unsupported.docs.camunda.io/8.1 for the staging build)
• Move "8.1 / 3.9.0" versions to the "Unsupported versions" sub-list in the top nav version selector, redirecting to unsupported.docs.camunda.io/8.1.
• Remove v8.1 & Optimize v3.9.0 from the sitemap ignore patterns.

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• There is no urgency with this change and can be released at any time.
• I'd like to merge in the next couple days, so that I can complete this archival work.

PR Checklist

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

• I included my new page in the sidebar file(s).
• I added a redirect for a renamed or deleted page to the .htaccess file.

• My changes require an Engineering review, and I've assigned the Engineerring DRI or delegate.
• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer.

• My changes require a docs infrastructure review.