introduce DEPRECATIONS.md to track deprecations

[tobybellwood]

This PR introduces a central file, DEPRECATIONS.md to track deprecations across Lagoon products.

I've opted to put it in the repository root, but we can discuss whether it should live in the docs?

Inspiration:

• https://platform.openai.com/docs/deprecations
• https://developers.google.com/maps/deprecations
• https://auth0.com/docs/troubleshoot/product-lifecycle/deprecations-and-migrations
• https://github.com/goreleaser/goreleaser/blob/main/www/docs/deprecations.md
• https://github.com/wix-incubator/wix-rest-docs/blob/master/guides/Deprecations.md