Remove deprecated horizontal regridding schemes unstructured_nearest and linear_extrapolate

[schlunma]

Description

In #2231 (in time for version 2.11.0), the regridding schemes unstructured_nearest and linear_extrapolate have been deprecated. Since we work on v2.13.0 now, these schemes can be removed for good.

Backwards-incompatible change

This PR removes two deprecated regridding schemes:

• unstructured_nearest: Please use the scheme nearest instead. This is an exact replacement for data on unstructured grids. ESMValCore is now able to determine the most suitable regridding scheme based on the input data.
• linear_extrapolate: Please use a generic scheme with reference: iris.analysis:Linear and extrapolation_mode: extrapolate instead. This is an exact replacement, e.g.:

preprocessors:
      regrid_preprocessor:
        regrid:
          target_grid: 2.5x2.5
          scheme:
            reference: iris.analysis:Linear
            extrapolation_mode: extrapolate

Related to #2231.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.19%. Comparing base (afcd1a0) to head (3ebdffc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2743      +/-   ##
==========================================
- Coverage   95.20%   95.19%   -0.01%     
==========================================
  Files         259      259              
  Lines       15221    15206      -15     
==========================================
- Hits        14491    14476      -15     
  Misses        730      730              

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[valeriupredoi]

2.13 is a bit aways but am happy to have this out now - saw the Tool PR earlier today too, many thanks, Manu 🍺