Include aggregator to ingest RSS (removed from Core in Drupal >= 10.0)

See https://www.drupal.org/project/aggregator.

Addresses websites throwing the following error:

[error]   (Currently using Removed core module You must add the following contributed module and reload this page.
 * Aggregator [1]

This module is installed on your site but is no longer provided by Core.
For more information read the documentation on deprecated modules. [2]

[1] https://www.drupal.org/project/aggregator
[2] https://www.drupal.org/node/3223395#s-recommendations-for-deprecated-modules
)