Rename the M2 theming API of Angular Material 18

Material 18 made Material 3 the default and renamed the Material 2 theming functions and palettes with an m2- prefix, such as mat.define-palette to mat.m2-define-palette and mat.$indigo-palette to mat.$m2-indigo-palette. Some old palette names, such as mat.$red-palette, are M3 palettes now, so this only applies to projects that are not on Material 18 yet. Also moves the M3 APIs of @angular/material-experimental to @angular/material. This is the M2 theming migration of the @angular/material 18 ng update.