Deprecate the https://github.com/wasp-lang/deploy-action

Reasons for the deprecation

• It's quite a basic action that doesn't bring that values beyond being "Wasp branded".
• The maintenance overhead: for each new provider it needs to be updated.
• It's hard to abstract away prerequisites for each deployment provider e.g. it needs to know about Fly CLI and Railway CLI install methods.
• It's quite easy to implement the same steps in a custom workflow (Install Wasp, run wasp deploy command)

Proposed course of action

• Update the README to explain how to replace the action with a custom workflow
• Update the action to output a warning (some people are using the action and they might end up using outdated practices over time!)
• Update the README of the repo to note the deprecation
• Archive the repo
• Update deployment docs to explain how to use wasp deploy in CI with GH Actions example