docs(repo): Add banner for impending retirement of code

[gibbardsteve]

📌 Add banner to README

✨ Summary

Retirement notice

📜 Changes Introduced

• documentation (docs:)
• Updates to tests and/or documentation

✅ Checklist

• Code is formatted using ruff format
• Imports are sorted using ruff
• Code passes linting with ruff and mypy
• Security checks pass using Bandit
• API and Unit tests are written and pass using pytest
• DocStrings follow Google-style and are added as per ruff recommendations
• Documentation has been updated if needed

🔍 How to Test

Manual review