Upgrade Mantid to v6.15

[ktactac]

Description of work:

Mantid release v6.15 is out, upgrading to use it here

Check all that apply:

• updated documentation
• Source added/refactored
• Added unit tests
• Added integration tests
• Verified that tests requiring the /SNS and /HFIR filesystems pass without fail

References:

• Links to IBM EWM items: 14566: Update lr_reduction to mantid v6.15
• Links to related issues or pull requests:

⚠️ Manual test for the reviewer

(Instructions for testing here)

Check list for the reviewer

• best software practices
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.15%. Comparing base (df01d17) to head (d142d52).
⚠️ Report is 1 commits behind head on next.

Additional details and impacted files

@@           Coverage Diff           @@
##             next     #123   +/-   ##
=======================================
  Coverage   85.15%   85.15%           
=======================================
  Files          22       22           
  Lines        2923     2923           
=======================================
  Hits         2489     2489           
  Misses        434      434           

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