Drupal 9 deprecated code report for Inline Entity Form

Some existing work:

• #3112427: \Drupal calls should be avoided in classes, use dependency injection instead in src/Plugin/Field/FieldWidget/InlineEntityFormBase.php
• #3121913: Type hints missing for some $form, $form_state variables
• #3121915: All dependencies must be prefixed with the project name, for example "drupal:"
• #3121922: t() calls should be avoided in classes, use \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t() instead

Also: #3127275: Use composer.json to define test dependencies

Removed #3112427: \Drupal calls should be avoided in classes, use dependency injection instead in src/Plugin/Field/FieldWidget/InlineEntityFormBase.php as it isn't strictly required.

With the referenced issues the codebase passes drupal-check:

$ drupal-check *
 24/24 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%
                                                                                                 
 [OK] No errors

Marking this 'postponed' until the other issues are committed.

I think this is RTBC now - the main sub issues have been committed and the only one left is more of a tidy-up thing, and drupal-check doesn't report any remaining bugs! Great work everyone!

I think so, yeah!