x11-toolkits/py-qt6-graphs: Fix PLIST_SUBs

Fix failure to package after 4b51612759d2. PYQT_PYQTVERSION should have
been PYQTGRAPHS_VERSION as these components don't always have the same
version. [1]

Fix over-agressive PYTHON_MAJOR_VER substitutions.

Add missing Qt components.

Reported by: pkg-fallout [1]
MFH: 2025Q1