Update to use the new hook.

[adam-vessey]

hook_field_widget_WIDGET_TYPE_form_alter() was deprecated in Drupal 9.2.0 and removed in Drupal 10; however, it was what this functionality was using.

It has been replaced with
hook_field_widget_single_element_WIDGET_TYPE_form_alter().

See: https://www.drupal.org/node/3180429

GitHub Issue: (link)

• Other Relevant Links (Google Groups discussion, related pull requests,
  Release pull requests, etc.)

What does this Pull Request do?

Fixes the hook naming, so the alt-text should be populated as it used to be.

What's new?

Nothing, just fixing.

How should this be tested?

See #783

Documentation Status

• Does this change existing behaviour that's currently documented? No.
• Does this change require new pages or sections of documentation? No.
• Who does this need to be documented for? Nobody, is fixing pre-existing functionality.
• Associated documentation pull request(s): ___ or documentation issue ___ N/A

Additional Notes:

Any additional information that you think would be helpful when reviewing this
PR.

Interested parties

Tag (@ mention) interested parties or, if unsure, @Islandora/committers

[rosiel]

Works as advertised.