Add project retirement notice to README

[rdeago]

Types of changes

• Bugfix
• Enhancement (new and/or improved behavior)
• Refactoring (no functional changes)
• Code style update (no functional changes)
• Dependencies added, updated, or removed
• Build related changes
• Repository infrastructure changes (e.g. changes to issue / PR templates)
• CI related changes
• Documentation update
• Other (please describe below)

Proposed changes / fixed issues

Added a retirement notice for the project due to inactivity and redundancy.

Checklist

• My contribution is my original work
• My contribution, or part of it, comes from projects with an MIT-compatible license AND I have updated the THIRD-PARTY-NOTICES file accordingly
• I have added and/or updated related documentation (if applicable)
• I have updated the "Unreleased changes" section in CHANGELOG.md according to the modifications I made

Other information

None.

[Copilot]

Pull request overview

Adds a prominent retirement notice to the top of the repository README to communicate project status and direct potential maintainers to the appropriate contact channel.

Changes:

• Added a “project retired” banner section at the top of README.md
• Added context for retirement and a suggested path for ownership transfer discussions

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