Make .conda the default format for future conda-build versions

[mbargull]

Checklist

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

What is the idea?

This is in the same vein as the suggestion for rattler-build: Make .conda the default format for rattler-build

Support for the .conda package format was introduced in 3.18.0 (2019-05-17) and has been the default format used at conda-forge since 2022-11-18:

• build and process .conda artifacts conda-forge/conda-forge.github.io#1586
• ENH make .conda the default conda-forge/conda-forge-ci-setup-feedstock#212

Why is this needed?

.conda uses Zstandard instead of bzip2 for compression and also separates metadata from packages' files making it more efficient in terms of package sizes, package extraction times, and metadata extraction.

What should happen?

We should consider marking a change of the default from the old .tar.bz2 format to .conda for a future conda-build version.

My suggestion would be "next release"+2, i.e., conda-build=24.7, as the earliest; it would be sensible in my opinion to schedule this change for sometime in the later half of this year.
The change itself is probably not very controversial since we'd only change the default, but still continue to support .tar.bz2.
But we should give enough time/notice such that downstream tooling can be updated beforehand.

Additional Context

Make .conda the default format for rattler-build