Use attributes for DateRecurInterpreter plugin discovery

Problem/Motivation

Since Drupal 10.2 we can use attributes for plugin discovery.

Proposed resolution

• Support attribute plugin discovery in 3.6.x
• Drop support for annotation plugin discovery in a later version

API changes

\Drupal\date_recur\Annotation\DateRecurInterpreter

Release notes snippet

DateRecurInterpreter plugins may now use attribute plugin discovery