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

> 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 https://github.com/AlexsLemonade/OpenScPCA-analysis/pull/1243#discussion_r2243007630_

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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`

Start by reading docs/ensuring-repro/managing-software/using-conda.md and templates/python/environment.yml, then search those files for osx-64 references and related platform recommendations. Update the documentation and template so they no longer recommend or retain osx-64, and verify that no such references remain in the specified spots.

Written by the indexing model from the issue text.