firestore: breaking changes

[Jonathan Amsterdam]

We've removed the DocumentRef.UpdateMap and DocumentRef.UpdateStruct methods. Their behavior was similar to DocumentRef.Set with a merge option, but different in confusing ways. The previous UpdatePaths method has been renamed to Update.

Change

    docref.UpdateMap(ctx, map[string]interface{}{"a.b", 1})

to

    docref.Update(ctx, []firestore.Update{{Path: "a.b", Value: 1}})

Change

    docref.UpdateStruct(ctx, []string{"Field"}, aStruct)

to

    docref.Update(ctx, []firestore.Update{{Path: "Field", Value: aStruct.Field}})