fix(gatsby-image): Update TypeScript types (#15313)

Author: brimtown

packages/gatsby-image/index.d.ts

@@ -27,10 +27,8 @@ export interface FluidObject {
 interface GatsbyImageProps {
   resolutions?: FixedObject
   sizes?: FluidObject
-  fixed?: FixedObject
-  fluid?: FluidObject
-  fixedImages?: FixedObject[]
-  fluidImages?: FluidObject[]
+  fixed?: FixedObject | FixedObject[]
+  fluid?: FluidObject | FluidObject[]
   fadeIn?: boolean
   title?: string
   alt?: string