Issue #3493095: fix SettingsForm errors

Chris Dartrequested to merge
issue/telephone_validation-3493095:3493095-the-settings-form into 8.x-2.x

• move SettingsForm to Drupal\telephone_validation\Form namespace • update routing to reference the new form class location • remove outdated src/SettingsForm.php to align with Drupal coding standards for form classes • update the reference to the PhoneNumber format function

Closes #3493095

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