cleanup: remove outdated 8.1 upgrade page and redirect to current upgrade page

[pepopowitz]

Description

Resolves #3607. Looks like a follow-up to #2515.

Completely removes the 8.1/upgrade page, and redirects that URL to the current version's upgrade page.

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.

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/).

• 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.