Change the Drupal core minimum version to Drupal 11.5.x

Problem/Motivation

Drupal 11.5.x will be the first branch where PHP attributes are used for defining routes, whenever those routes are used for a controller page or a form.

Proposed resolution

Change the Drupal core minimum version to Drupal 11.5.x when Drupal 11.5.x will have its first pre-release.