Updated version number for MSlice 2.12

[SilkeSchomann]

Description of work

The latest released version of MSlice is version 2.12. This PRs updates the Conda recipe for Mantid to allow using the latest version of MSlice.

There is no associated issue.

To test:

Run the following command to satisfy yourself that pinning mslice=2.12 will pick up the latest MSlice version:
conda create -n mslice_nightly_test -c mantid mslice=2.12 --dry-run

This does not require release notes because all release notes for MSlice have already been added beforehand.

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[sf1919]

Fairly straight forward change required for release.