ServedFileForm incompatible with D10 [#3392229]

Problem/Motivation

HTTP 500 response when visiting admin/config/system/served_files/add:

Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: You have requested a non-existent service "app.root". in Drupal\Component\DependencyInjection\Container->get() (line 157 of core/lib/Drupal/Component/DependencyInjection/Container.php).

Drupal\serve_plain_file\Form\ServedFileForm::create(Object) (Line: 28)
Drupal\Core\DependencyInjection\ClassResolver->getInstanceFromDefinition('Drupal\serve_plain_file\Form\ServedFileForm') (Line: 210)
...

Steps to reproduce

See above (in D10)

Proposed resolution

Fix code, while temporarily keeping backward compatibility.

Remaining tasks

User interface changes

Review, commit.

Issue fork serve_plain_file-3392229

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3392229-servedfileform-D10 changes, plain diff MR !4
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