Update to mamba 2.1.0

[jjerphan]

I wonder whether this could be automated.

I am only updating mamba because conda 25.4.0 is not out yet.

cc @hmaarrfk.

[hmaarrfk]

i don't have time to think about how to automate this. We have had many issues with "other projects" breaking the installer.

Generally, i think that conda moves too fast to really have meaningful releases here.

The main thing is that "the conda command should work"

If it does, and is slightly out of date, that is better than "not working because we didn't have time to test a release".

Feel free to make a PR to automate the "bumping of versions", but I don't want to "automate the making of releases" since those typically require "human thought" to assess quality.

[jjerphan]

I agree.

Closing in preference of #777 since the scope changed.

[hmaarrfk]

for others, we try to add testing to the build process, but it doesn't really mimic a "human" using the tool, rather it mimics our "expert understanding of how the tool should work in a CI".