docs migration - Log Levels (library authors)

[heckj]

Source: documentation/server/guides/libraries/log-levels.md
Proposed disposition: Deprecate — redirect to swift-log documentation
Proposed destination: https://swiftpackageindex.com/apple/swift-log/documentation/logging/001-choosingloglevels
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 swift-log docs
• Submit redirect PR to swift-org-website