refactor: change JSONFile to private instead of internal

This is so that this can be used in the Universal repo which is not failing because the type is not available.

https://app.circleci.com/pipelines/github/angular/universal/4483/workflows/25535db2-4a9c-43b0-84a5-3a55071013c6/jobs/20263
(cherry picked from commit e763db8)

packages/schematics/angular/utility/json-file.ts

@@ -22,7 +22,7 @@ import {
 export type InsertionIndex = (properties: string[]) => number;
 export type JSONPath = (string | number)[];
 
-/** @internal */
+/** @private */
 export class JSONFile {
   content: string;