docs migration - Swift Concurrency Adoption Guidelines (library authors)

[heckj]

Source: documentation/server/guides/libraries/concurrency-adoption-guidelines.md
Proposed disposition: Deprecate — redirect to Swift Migration Guide content for concurrency
Proposed destination: https://www.swift.org/migration/documentation/swift-6-concurrency-migration-guide/enabledataracesafety/
Reviewer: @swiftlang/server-workgroup

Checklist

• Create a GitHub issue to track this migration
• Determine disposition (suggested: Deprecate) and confirm destination
• Add redirect_to: in the swift.org source file's front matter pointing to the migration guide
• Submit redirect PR to swift-org-website

[heckj]

PR pending for redirect: swiftlang/swift-org-website#1323