Use opencv headless

[nfahlgren]

Describe your changes
Alter PlantCV dependencies from PyPI and Conda-Forge to use the OpenCV headless package instead of the full version since we do not use OpenCV GUI features.

Type of update
Is this a: improvement for efficiency

Associated issues
#1515

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[deepsource-io]

Here's the code health analysis summary for commits 432fcd2..e22c2d5. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	⚠️ Artifact not reported	Timed out: Artifact was never reported	View Check ↗

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