Fix CI tests

[RemiLehe]

The CI tests are currently failing on the master branch of slaclab/lume-model:
https://github.com/slaclab/lume-model/commits/master/

This PR introduces several changes to fix the CI tests:

• Use botorch<0.11:
  This is because botorch 0.11 is only compatible with Python>=3.10, while the CI runs on Python 3.9

• Use keras<3:
  This is because the CI tests download keras 3 by default, while the .keras files saved in the lume-model repo (e.g. here) are not compatible with keras 3. (Presumably because they were created some time ago.)

• Replace mamba build with conda mambabuild (which requires boa)

[RemiLehe]

Hm, this is strange:

• the ubuntu CI tests pass now
• the macOS CI tests do not pass. And I am unable to reproduce the problem locally (on a MacBook)

[RemiLehe]

As discussed offline, I remove the MacOS tests for now.