Use Viridis as colour palette for Ert and Everest

[berland]

Issue
Resolves #13288

Approach
Calculate hex values via Matplotlib, and pick discrete colours in a specific fashion to maximise separation.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.36%. Comparing base (d5f13f9) to head (cb1b66c).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13414      +/-   ##
==========================================
- Coverage   89.99%   85.36%   -4.64%     
==========================================
  Files         460      460              
  Lines       32320    32332      +12     
==========================================
- Hits        29087    27599    -1488     
- Misses       3233     4733    +1500     

Flag	Coverage Δ
cli-tests	37.23% <ø> (+<0.01%)	⬆️
fuzz	44.06% <ø> (-0.02%)	⬇️
gui-tests	?
performance-and-unit-tests	78.09% <ø> (+0.02%)	⬆️
test	45.62% <ø> (+<0.01%)	⬆️

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see 70 files with indirect coverage changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 36 untouched benchmarks

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_{Comparing berland:viridis (ae9f9b8) with main (beb9d13)}