Add box-plots to standard deviation plot

[xjules]

Issue
Continuation of #8565

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

• 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 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

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: Create Backport PR to latest release

[xjules]

Still getting different results on the ubuntu runner, even with the same matplotlib & python versions
ubuntu

RHEL8

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.41%. Comparing base (f6b86c6) to head (2f7723f).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ert/gui/plottery/plots/std_dev.py	91.17%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8833      +/-   ##
==========================================
- Coverage   91.41%   91.41%   -0.01%     
==========================================
  Files         343      344       +1     
  Lines       21086    21122      +36     
==========================================
+ Hits        19276    19308      +32     
- Misses       1810     1814       +4     

Flag	Coverage Δ
cli-tests	39.63% <0.00%> (+<0.01%)	⬆️
gui-tests	73.55% <91.17%> (+0.04%)	⬆️
performance-tests	50.16% <2.94%> (-0.03%)	⬇️
unit-tests	80.09% <91.17%> (+0.20%)	⬆️

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