Group-Based Optimization of Potent and Cell-Active Inhibitors of the von Hippel–Lindau (VHL) E3 Ubiquitin Ligase: Structure–Activity Relationships Leading to the Chemical Probe (2 S ,4 R )-1-(( S )-2-(1-Cyanocyclopropanecarboxamido)-3,3-dimethylbutanoyl)-4-hydroxy- N -(4-(4-methylthiazol-5-yl)benzyl)pyrrolidine-2-carboxamide (VH298)