pin mantid to 6.11.0.3 stable for release

[walshmm]

As title states.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.98%. Comparing base (b459990) to head (5d0e650).
Report is 48 commits behind head on next.

Additional details and impacted files

@@           Coverage Diff           @@
##             next     #537   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files          68       68           
  Lines        5152     5152           
=======================================
  Hits         4945     4945           
  Misses        207      207           

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