Deprecate CONDA_USES_MAMBA

[astrojuanlu]

Projects willing to use conda on Read the Docs now have an alternative way of doing it that doesn't require our intervention:

version: 2

build:
  os: "ubuntu-20.04"
  tools:
    python: "mambaforge-4.10"

conda:
  environment: environment.yml

Despite some minor confusion (#8595) this is working already for at least two projects, and it accelerated the total build time by about 5x, as expected.

We could start by removing mentions of it on our documentation: https://docs.readthedocs.io/en/stable/feature-flags.html?highlight=mamba#available-flags and https://docs.readthedocs.io/en/stable/guides/conda.html?highlight=mamba#troubleshooting, pretend that it doesn't exist, and instead tell users to pick the right python interpreter.

Additionally, we could email projects that have the flag enabled (can't check how many of them they are at the moment) and tell them the new solution. But it would surely cause a bit of churn, and I'm not sure if it's worth it.

[humitos]

I'm 👍🏼 on this. We could add a FAQ entry "How I can use Mamba instead of Conda?" that points to the build.tools.python: mambaforge option.

[astrojuanlu]

Welp, it's only 31 projects! I expected many more.

In [37]: Feature.objects.get(feature_id=Feature.CONDA_USES_MAMBA).projects.count()
Out[37]: 31

[astrojuanlu]

Migration ongoing using @readthedocs-assistant for projects with this flag 🚀

[humitos]

This work is already done. The only missing part is tracking projects that haven't merged the PR that our assistant opened. We have that data in a different document.