Up the max num of batches to 10

[eqbech]

Issue
Resolves #13459

[berland]

This also affects Ert, in that you can now plot up to 10 ensembles at the same time. This makes sense, but implies that the commit message can mention ensembles instead of or in addition to batches.

[eilskra]

It should be conditional based on plot type. We don't want 10 batches pre-selected for some plots, e.g with summary data, ref #13458

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.62%. Comparing base (8d253e0) to head (0024ed5).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13550      +/-   ##
==========================================
- Coverage   89.63%   89.62%   -0.02%     
==========================================
  Files         460      460              
  Lines       32519    32519              
==========================================
- Hits        29150    29146       -4     
- Misses       3369     3373       +4     

Flag	Coverage Δ
cli-tests	37.01% <0.00%> (ø)
fuzz	43.97% <100.00%> (ø)
gui-tests	63.70% <100.00%> (-0.01%)	⬇️
performance-and-unit-tests	78.03% <100.00%> (-0.01%)	⬇️
test	45.63% <100.00%> (ø)

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Files with missing lines	Coverage Δ
...t/gui/tools/plot/plot_ensemble_selection_widget.py	89.28% <100.00%> (ø)

... and 1 file with indirect coverage changes