Change sharded repodata to enabled by default

[danyeaw]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

Right now sharded repodata requires a step to turn it on for the beta:

conda config --set plugins.use_sharded_repodata true

or

export CONDA_PLUGINS_USE_SHARDED_REPODATA=true

Once we are ready to make it generally available, we should make it enabled by default.

Why is this needed?

Users can get the speed enhancements by default. If a user decides they don't want to use it, then they can configure it off.

What should happen?

Change the conda_settings() plugin primitive from False to True. Ensure the release is coordinated and announced.

Additional Context

No response

[dholth]

See also #842 which suggests an issue with authenticated channels.