fix(gatsby): use correct types for the response headers (#35633)

Author: feedm3

packages/gatsby/index.d.ts

@@ -165,7 +165,7 @@ export type GetServerDataProps = {
  */
 export type GetServerDataReturn<ServerDataType = Record<string, unknown>> =
   Promise<{
-    headers?: Map<string, unknown>
+    headers?: Record<string, unknown>
     props?: ServerDataType
     status?: number
   }>