Don't recommend use of osx-64 platform for conda-lock

[sjspielman]

I did a bit of testing and it seems like most of the failures in builds are for osx-64, which is mostly deprecated even by OS standards, so I think the best option might be to start just dropping that and keeping with more recent versions of packages (I don't think we should pin much in the environment.yml files themselves, as it often makes updates and changes much harder)

Originally posted by @jashapiro in #1243 (comment)

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This issue tracks updating docs & templates to rip out osx-64. Spots will include:

• docs/ensuring-repro/managing-software/using-conda.md
• templates/python/environment.yml

[jashapiro]

While we do this, we may well want to update some of the minimum versions of the default packages, just for consistency. For example, conda-lock is up to 3.0 which introduces some format changes. I think still backward compatible, but we might as well get ahead of it.