y-axis spine z-order too low in plot_evoked

[drammock]

The GFP curve in mne.viz.plot_evoked is plotted above the y-axis spine. But that is not limited to the GFP-only case, the channels are also plotted above the y-axis in other evoked plots.

I guess we just need to adjust the zorder of the axis spines.

Originally posted by @mistraube in https://github.com/mne-tools/mne-python/issues/13481#issuecomment-3538339605