Set iris.FUTURE flags in one place

[bouweandela]

Description

There is a new iris.FUTURE flag in iris 3.11.1 and it would be good to set that because not doing that will cause a performance degradation. Moved all the flags to the config module to keep them in one place.

This also fixes the failing tests on main: https://app.circleci.com/pipelines/github/ESMValGroup/ESMValCore/12309/workflows/ec7ef641-9cde-4354-87c6-c70fc3e0244b

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[bouweandela]

I'll keep this in draft until we can test with iris 3.11.1 and cf_units 3.3.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.96%. Comparing base (23400b1) to head (1adfcb8).
Report is 59 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2622   +/-   ##
=======================================
  Coverage   94.96%   94.96%           
=======================================
  Files         253      253           
  Lines       14696    14701    +5     
=======================================
+ Hits        13956    13961    +5     
  Misses        740      740           

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[schlunma]

Thanks Bouwe!