Cleanup/resolve deprecations
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What this MR does / why does DuMux need it:
Resolves deprecation warnings introduced by !3202 (merged).
Extrusion::area(scvf)
and BoxGeometryHelper.scvfArea/Volume(corners_)
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CHANGELOG.md
.\n
character. Make sure there is no \ No newline at end of file
comment in "Changes" of this MR.generate_example_docs.py
)