Fix/seuratv5 fetchdata layer

[mgarber]

•	Updates VlnPlot.xlabel() for SeuratObject v5 by switching FetchData(slot=) → FetchData(layer=) (with backward compatibility).
•	Fixes ggplot2 facet_grid API change (use rows= / cols= instead of deprecated facets=).
•	Keeps function behavior the same aside from compatibility fixes.