Move preprocessor output filename template to new configuration

[bouweandela]

Description

Move the preprocessor output filename template to the new configuration.

Upgrade instructions

If you have defined projects in config-developer.yml that are not in the default config-developer.yml file shipped with ESMValCore, then you will need to move the value of 'output_file' to

projects:
  <insert project name here>:
    preprocessor_filename_template: <insert output_file value here>

Closes #2919

Link to documentation:

• https://esmvaltool--2923.org.readthedocs.build/projects/ESMValCore/en/2923/quickstart/configure.html#project-specific-configuration
• https://esmvaltool--2923.org.readthedocs.build/projects/ESMValCore/en/2923/quickstart/configure.html#config-preprocessor-filename-template

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.59%. Comparing base (dc340e9) to head (d8df0ac).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2923      +/-   ##
==========================================
+ Coverage   95.57%   95.59%   +0.01%     
==========================================
  Files         266      266              
  Lines       15520    15556      +36     
==========================================
+ Hits        14834    14870      +36     
  Misses        686      686              

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[valeriupredoi]

looks very good - I am wondering if we want to start thinking about allowing preprocessor files that are not netCDF4 for regular cases ie allow a bit of flexibility when it comes to appending that .nc extension - probably, but in the future, and definitely outside the scope of this PR. Cheers @bouweandela 🍻

[bouweandela]

I considered the option of adding a DataSink similar to the esmvalcore.io.protocol.DataSource but then for writing data to, so you could write your data as zarr to an object store, but the difficulty is that diagnostic scripts wouldn't support this, so that would require some further work on the ESMValTool side as well, especially for languages other than Python, and I'm not sure how useful this is anyway as diagnostic scripts are typically expected to work with small amounts of data after the heavy lifting has been done by the preprocessor functions.