Fix CMIP5 MRI-ESM1 grid

[bouweandela]

Description

Replace the rotated pole coordinates by index coordinates for CMIP5 MRI-ESM1 fgco2
to avoid the regridder picking up the wrong grid.

See ESMValGroup/ESMValTool#3916 (comment) and ESMValGroup/ESMValTool#3916 (comment)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.08%. Comparing base (06ce74d) to head (6b1695a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2670   +/-   ##
=======================================
  Coverage   95.08%   95.08%           
=======================================
  Files         255      255           
  Lines       14900    14921   +21     
=======================================
+ Hits        14168    14188   +20     
- Misses        732      733    +1     

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[bouweandela]

This may not be necessary if #2672 works out.