In ca.state plot replace deprecated norm kwarg for colorbar

Description

With recent matplotlib versions, the norm kwarg is not supported anymore for colormaps, see the log warning:

MatplotlibDeprecationWarning: The 'norm' parameter to Colorbar has no effect because it is overridden by the mappable; it is deprecated since 3.3 and will be removed two minor releases later.

Proposal

Remove the norm kwarg and device a different way of setting the colorbar norm.

How to test the implementation?

Look at the resulting figures and check that the norm is set correctly since we do not have automated tests for the plot function.

Related issues

Meta-task: #...