MAINT: Improve qdarkstyle logic

[larsoner]

1. Bump Qt binding minimum to 5.15, released almost 4 years ago. This in principle could be 6.0 but I know conda-forge doesn't have suitable bindings for it yet (PySide6 is in progress at least but does not provide a builtin event loop like PyQt6 does so matplotlib etc. do not behave as expected for plain python + plt.ion() users).
2. Simplify logic of stylesheet setting by removing some version conditionals and short-circuiting when possible to reduce indentation. We now only need to warn when qdarkstyle < 3.2.3 (3.2.3 came out Nov '23) is used with a Qt6 variant

[larsoner]

This fixes a failure currently in main so I'll mark for merge-when-green but happy to revise if you review and see issues @drammock