Standardize Rector rules as part of Drupal core deprecations

Background:
It took only 4 months to create Rector rules for the top 50 Drupal core deprecations, covering over 72% of core deprecations that are used by contrib modules.

Drupal Rector bot was added and was an immense help to deliver ready-made patches for D9 readiness directly to modules maintainers.

Purpose of this BOF:

Moving forward, can we make it an official part of the process, to release a Rector rule along with each core deprecation? That would allow contrib modules to become evergreen and always ready for the next major Drupal version.