Issue #3506177 by juandhr: Modify support mentions for Drupal core's search in README

Issue #3506177 by juandhr: Modify support mentions for Drupal core's search in README

Replace lines 26 and 27 in README.md:

No other modules are required; we're supporting Drupal Core's search as a source for creating facets.

With:

Search API is required: Facets only supports Search API as a source for creating facets. Drupal Core's search is no longer supported as a source for facets.

Closes Issue #3506177