Summary

Add figure.title_horizontalalignment and figure.title_verticalalignment rcParams so that users can control the default alignment of figure titles via rc settings. Previously, only fontsize (figure.titlesize) and fontweight (figure.titleweight) were configurable via rcParams; horizontal/vertical alignment were hardcoded to "center" / "top".

This PR addresses #24090 and uses the naming discussed in the issue: figure.title_horizontalalignment and figure.title_verticalalignment (underscored per matplotlib convention).

Proposed changes

• Added validate_suptitle_ha validator + two new rcParam entries with defaults matching the old hardcoded values ("center" / "top")
• Updated figure._suplabels() to resolve ha/va from rcParams when the key is set in the info dict
• Added commented-out default values to matplotlibrc
• Two new tests: rcParams alignment applies, kwarg overrides work
• Updated docstring to reference new rcParams

Additional context

import matplotlib.pyplot as plt

plt.rcParams[\"figure.title_horizontalalignment\"] = \"left\"
plt.rcParams[\"figure.title_verticalalignment\"] = \"bottom\"

fig, ax = plt.subplots()
fig.suptitle(\"Left-aligned title\")  # Uses rcParams defaults
fig.suptitle(\"Override\", ha=\"right\")  # Explicit kwarg still wins

Backwards compatible — defaults match the old hardcoded values.

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