Deprecate file_managed_file_submit form submit handler and move to \Drupal\file\Element\ManagedFile

I have applied the MR on a fresh Drupal 11.x site and confirmed the following through the UI:

• Created a content type with a File field and tested upload and remove functionality. Everything worked as expected.
• Repeated the test with an Image field — both upload and remove actions work correctly.
• Verified behavior for single and multi-value fields.
• Tested required field validation by removing the uploaded file — validation triggers as expected.
• Confirmed no errors in the PHP logs or JavaScript console.
• Verified that file_managed_file_submit() is now deprecated and the logic is moved to \Drupal\file\Element\ManagedFile::submit(), maintaining BC correctly.
• Everything works as expected, and the code looks clean and standards-compliant.

Marking this issue RTBC.

Thanks!