Deprecate and make use of PluginFormInterface

Purge uses a custom form implementation to describe forms which provide configuration for plugins. It would be great if Purge could adopt the standard approach to plugin forms for DX.

Problem/Motivation

Purge processor, purger, and queuer plugins implement a custom form plugin system with the "configform" annotation:

\Drupal\purge_queuer_coretags\Plugin\Purge\Queuer\CoreTagsQueuer:

/**
 * @PurgeQueuer(
    [...]
 *   configform = "\Drupal\purge_queuer_coretags\Form\ConfigurationForm",
 * )
 */

This implementation should be replaced by the core plugin form system outlined in Plugins can provide multiple forms.

Example, \Drupal\content_moderation\Plugin\WorkflowType\ContentModeration:

/**
 * @WorkflowType(
     [...]
 *   forms = {
 *     "configure" = "\Drupal\content_moderation\Form\ContentModerationConfigureForm",
 *     "state" = "\Drupal\content_moderation\Form\ContentModerationStateForm"
 *   },
 * )
 */

Where many plugin types implement a default "configure" form, via \Drupal\Core\Plugin\PluginWithFormsTrait.

Proposed resolution

Deprecate the existing configform implementation. If plugins choose to implement PluginFormInterface, then use that instead.