examples: update geopandas usage

[maurerle]

Changes proposed in this Pull Request

This updates the examples to work with latest package versions.
Geopandas does not vendor naturalearth dataset anymore.
This is instead downloaded from "https://www.naturalearthdata.com/http//www.naturalearthdata.com/download/110m/cultural/ne_110m_admin_0_countries.zip".

Fixes cartopy outline_patch deprecation as mentioned here: https://cartopy.readthedocs.io/stable/whatsnew/v0.18.html#deprecations

Fixes Shapely unary_union and previous cascaded_union deprecation: https://shapely.readthedocs.io/en/stable/reference/shapely.unary_union.html

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.