Re-enable MacOS conda build

[naegelejd]

The MacOS conda build is disabled in the CI pipeline because:

1. The macos-13 runner was deprecated. We need to upgrade to macos-14 or macos-15
2. We also need to publish conda packages for ismrmrd and ismrmrd-python for the corresponding MacOS platform (ismrmrd is still using macos-13), i.e. update the CI runners in those repositories.
3. xtensor-fftw does not appear to have a conda package for macos-14 or newer, so we'll need to solve that.