Enable v3 repodata

The v3 CEPS have all been accepted by now. Especially the [CEP 48](https://github.com/conda/ceps/blob/main/cep-0048.md) which describes a backwards-compatible update strategy for repodata `v3` has been accepted.

I would therefore like to propose to move forward with `v3` adoption for `conda-forge`. At prefix.dev we are happy to help with this, but I assume that also work has to be done on Anaconda's side to enable this.

@lwinfree @conda-forge/core

Start with CEP 48 to understand the backwards-compatible repodata v3 adoption strategy, then determine what work is required on the conda-forge and Anaconda sides. Done means v3 adoption is enabled for conda-forge, but the issue does not name files, tests, or a specific implementation entry point.

Written by the indexing model from the issue text.