Fold drupal_get_updaters() into Updater class, deprecate drupal_get_updaters()

Problem/Motivation

#1577902: [META] Remove all usages of drupal_static() & drupal_static_reset() wants us to get rid of drupal_static().

#3015538: [META] Deprecate contents of common.inc wants us to deprecate functions in common.inc.

Thus here we feet two birds with one seed.

Proposed resolution

Move the functionality of drupal_get_updaters() into a static method on Drupal\Core\Updater\Updater. Let's call it getUpdaterRegistry().

Instead of using drupal_static() to cache in memory, we'll add a static property to Drupal\Core\Updater\Updater.

Deprecate drupal_get_updaters() in favor of Updater::getUpdaterRegistry().

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet