Issue #3612285: Drop Drupal 10 support and remove the legacy requirements hook

Max Pogonowskirequested to merge
issue/route_basic_auth-3612285:3612285-drop-drupal-10 into 2.x

With a minimum of Drupal 11.3 the hook_runtime_requirements() hook and the RequirementSeverity enum are always available, so the legacy hook_requirements() implementation and its BC shim are no longer needed.

Closes #3612285

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