Use PHP attributes instead of annotations for plugins

Problem/Motivation

PHP introduced attributes with the release of 8.0. Drupal began supporting using attributes instead of annotations with version 10.2, which required a minimum of PHP 8.1. Linkchecker's .info.yml file says it does not support Drupal < 10.2. Parsing Doctrine annotations is significantly slower than using native attributes. It is trivial to support both going forward (though Drupal 12 will likely drop support for annotations). Therefore, there is no reason why we shouldn't move ahead with migration.

Proposed resolution

Add attributes for plugins, and register them with plugin managers.

User interface changes

None

API changes

None (until Drupal 12, anyway)

Data model changes

None