[WIP] Remove deprecated Matplotlib stem() parameter - Fixes #1314

[lwgray]

Description

This PR fixes a TypeError that occurs when using newer versions of Matplotlib by removing the deprecated use_line_collection parameter from stem plots in the regressor module.

Breaking Change ⚠️

This change requires updating Yellowbrick's minimum Matplotlib dependency to 3.6.0 or later, as this version made LineCollection the only implementation for stem plots.

TODO Before Merge

• [ ] Audit all Matplotlib API usage throughout Yellowbrick for other 3.6.0+ incompatibilities
• [ ] Update minimum Matplotlib version in setup.py and requirements.txt
• [ ] Update documentation to reflect new minimum version requirement
• [ ] Add migration notes to changelog

Testing Done

• Verified CooksDistance visualizations work correctly with Matplotlib 3.6.0+
• Tests pass locally with updated Matplotlib version

Related Issues

• Fixes #1314

I'll remove the WIP tag once we've completed the Matplotlib version upgrade across the entire package.