ci: improve angular.io deployment process for the stable branch (#43963)

Previously, the stable branch was always deployed to the
`v<X>-angular-io-site` Firebase site, which was connected to the
`angular.io` domain. Whenever a new major version was released (and
became the new stable version), the `angular.io` domain had to be
disconnected from the previous Firebase site and be connected to the new
`v<Y>-angular-io-site` Firebase site. This was a manual process that
involved making changes in the Firebase console and the DNS records.

This commit is part of a new process that reduces the manual steps as
follows:
- A new `stable-angular-io-site` Firebase site is created.
- The `angular.io` domain will be connected to that new Firebase site.
- When deploying from the stable branch, we will deploy to both
  `stable-angular-io-site` and `v<X>-angular-io-site`. In addition, the
  deployment to `v<X>-angular-io-site` will update the Firebase config
  file to redirect to `angular.io`.
- When a new major version is released, we will start deploying from the
  new stable branch to `stable-angular-io-site`, but there will be no
  need to connect/disconnect the `angular.io` domain. Also,
  `v<X>.angular.io` will stop redirecting to `angular.io` by means of
  updating the Firebase config file (without requiring changes in the
  Firebase console or DNS).

PR Close #43963