build: update ng-packagr to 12.1 stable version

(cherry picked from commit 7d2ed2a)

Author: alan-agius4

packages/schematics/angular/utility/latest-versions.ts

@@ -19,5 +19,5 @@ export const latestVersions = {
   // non-experimental, they will always have the same version.
   DevkitBuildAngular: '~' + require('../package.json')['version'],
 
-  ngPackagr: '^12.1.0-next.0',
+  ngPackagr: '^12.1.0',
 };