update plot_euler_number.py for maplotlib 3.6 compatibility

[grlee77]

Description

closes #6521

As of matplotlib 3.6, passing kwargs to the figure gca method is no longer supported

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8
• Descriptive commit messages (see below)

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.
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  comment: @meeseeksdev backport to v0.19.x
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[grlee77]

@scikit-image/core : please approve/merge so CI will be back to a working state

[rfezzani]

👍

[grlee77]

Thanks @rfezzani!