BLD: Update minimum Python in meson.build to 3.12 - #32470

PR summary

The minimum Python was raised to 3.12 in #31740, but the version
check on find_program in meson.build still says '>= 3.11'.
pyproject.toml asks to keep the two in sync ("Also keep in sync with
find_program of meson.build").

When building through meson-python, find_program returns the build
interpreter (the "python" entry of meson-python's native file), so
this check is what rejects an unsupported Python at meson setup. On
Python 3.11 it now stops with "found 3.11.x but need: '>= 3.12'"
instead of passing the check.

AI Disclosure

Claude Code (Claude Opus 5.5 (high) ) flagged this while auditing docs and reviewing codebase.

PR quality check

• Use an expressive title, e.g. "Fix title font property precedence"
• [N/A] New and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New features and API changes have release notes
• [N/A] Documentation complies with general and docstring guidelines