Using @FieldWidget annotation for plugin with ID readonly_field_widget is deprecated [#3621874]

Problem/Motivation

Warning message when testing an unrelated module:
Using @FieldWidget annotation for plugin with ID readonly_field_widget is deprecated and is removed from drupal:13.0.0. Use a Drupal\Core\Field\Attribute\FieldWidget attribute instead. See https://www.drupal.org/node/3395575

Steps to reproduce

Run tests of a module on D11.4/PHP 8.3 or similar.

Proposed resolution

I'll create an MR and push changes. That attribute only appeared in D10.3 so I'll have to provide both the attribute and the annotation.

Remaining tasks

Test BC.

Issue fork readonly_field_widget-3621874

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3621874-using-fieldwidget-annotation changes, plain diff MR !18
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