Use old upload endpoint until quetz release

[simonbohnen]

• Use old upload endoint
  • The new upload endpoint doesn't work correctly yet (see Upload endpoint does not trigger index update quetz#590). We need to wait for a quetz 0.5.1 release, which includes the fixes in Fix upload endpoint quetz#597 and Fix upload endpoint quetz#594).
  • Until that release is out, we should use the old endpoint.
• Implement quetz README script
  • The client is validated by implementing the steps from the quetz README
  • You can find the corresponding run here.
• Add compatibilty notice to README
• Remove quetz as a requirement in environment.yml

[janjagusch]

@simonbohnen, all your PRs in quetz were merged. Instead of merging this PR, should we try to release a new Quetz version?

[simonbohnen]

@simonbohnen, all your PRs in quetz were merged. Instead of merging this PR, should we try to release a new Quetz version?

Sounds good, I'll wait for the new quetz release 👌🏼