CI updates and cleanup

[altheaden]

This PR adds a few CI updates we've implemented in other repos recently:

1. I updated the default python version in various places to 3.13, and changed this version from a magic number to a variable in a few places (for consistency).
2. I enabled auto-update-conda which we had disabled previously thinking it was causing trouble, but it wasn't.
3. I removed the caching step for creating a conda environment - it doesn't appear to be saving any time and we don't consider it worth keeping.

Checklist

• Documentation has been built locally and changes look as expected
• Document (in a comment titled Testing in this PR) any testing that was used to verify the changes

[altheaden]

Testing

I pushed this to my main branch on my fork and ran all the CI tests, everything passed and the docs built as expected (as far as I can tell).

@xylar I updated 3.10 -> 3.13 in a few places besides the docs, so let me know if that looks OK to you.

[altheaden]

@xylar I don't have permission to re-run jobs but I think the failed job is due to the CI issues we've been experiencing lately.

[xylar]

Looks great!