Update to new ESMValTool logo

[zklaus]

Description

Updates to the new ESMValTool logo in README.md and documentation to sync up with ESMValTool documentation.

Closes #1368
Link to documentation: https://esmvaltool--1374.org.readthedocs.build/projects/ESMValCore/en/1374/

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[bettina-gier]

Looks good!

[codecov]

Codecov Report

Merging #1374 (d9f87d5) into main (6495cc3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1374   +/-   ##
=======================================
  Coverage   88.39%   88.39%           
=======================================
  Files         195      195           
  Lines        9958     9958           
=======================================
  Hits         8802     8802           
  Misses       1156     1156           

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