message: docs: deprecate `@angular-devkit/build-optimizer`
  The entire package is deprecated in v13. Deprecation message is mostly copied from the package's [README](https://github.com/angular/angular-cli/blob/13.0.0-next.9/packages/angular_devkit/build_optimizer/README.md), with just some minor edits.
  PR Close
diff: @@ -489,6 +489,12 @@ This section contains a complete list all of the currently deprecated CLI flags.
  | `showCircularDependencies` | <!--v12--> v14 | The recommended method to detect circular dependencies in project code is to use either a lint rule or other external tooling. |
  | Protractor builder | <!--v12--> v14 | Deprecate as part of the Protractor deprecation. |
  +### @angular-devkit/build-optimizer
  +
  +The entire NPM package is deprecated. It has always been experimental (never hit `1.0.0`) and has
  +been an internal package for the Angular CLI. All the relevant functionality has been moved to
  +`@angular-devkit/build-angular`.
  +
  {@a removed}
  ## Removed APIs
  