Add fix for EC-Earth3-Veg tos calendar

[bouweandela]

Description

Converts the EC-Earth3-Veg tos calendar to proleptic_gregorian to make it possible to concatenate the historical experiment with the ssp126 experiment.

See #1731

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[codecov]

Codecov Report

Merging #2100 (eb00840) into main (41f46ab) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2100   +/-   ##
=======================================
  Coverage   93.08%   93.09%           
=======================================
  Files         237      237           
  Lines       12793    12795    +2     
=======================================
+ Hits        11909    11911    +2     
  Misses        884      884           

Impacted Files	Coverage Δ
esmvalcore/cmor/_fixes/cmip6/ec_earth3_veg.py	100.00% <100.00%> (ø)

[valeriupredoi]

nice one! 🍺