Update mslice version to allow new nightlies to be installed

[thomashampson]

Description of work

Update mslice pin to 2.15.* to allow mslice nightly to be installed.

To test:

On Linux:

mamba create -n mslice-test
mamba activate mslice-test
mamba install -c mantid-test/label/mslice-pin-update -c mantid/label/nightly mantidworkbench mslice --dry-run

Check versions of mantidworkbench and mslice:

• mantidworkbench should be 6.15.20260325.1402.dev2
• mslice should be 2.15.dev6

---

Reviewer

Your comments will be used as part of the gatekeeper process. Comment clearly on what you have checked and tested during your review. Provide an audit trail for any changes requested.

As per the review guidelines:

• Is the code of an acceptable quality? (Code standards/GUI standards)
• Has a thorough functional test been performed? Do the changes handle unexpected input/situations?
• Are appropriately scoped unit and/or system tests provided?
• Do the release notes conform to the guidelines and describe the changes appropriately?
• Has the relevant (user and developer) documentation been added/updated?
• If the PR author isn’t in the mantid-developers or mantid-contributors teams, add a review comment rerun ci to authorize/rerun the CI

Gatekeeper

As per the gatekeeping guidelines:

• Has a thorough first line review been conducted, including functional testing?
• At a high-level, is the code quality sufficient?
• Are the base, milestone and labels correct?

[github-actions]

Unit test results

2 863 tests   2 863 ✅  10h 48m 22s ⏱️
    1 suites      0 💤
    1 files        0 ❌

Results for commit 7ef5036.

[github-actions]

System test results

808 tests   792 ✅  3h 0m 35s ⏱️
 11 suites   16 💤
 11 files      0 ❌

Results for commit 7ef5036.

[jhaigh0]

tested and looks correct

failing test unrelated