Improve the UI strings used by UserRestrictionAddForm

Problem/Motivation

The UserRestrictionAddForm class uses the following code.

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state): void {
    parent::submitForm($form, $form_state);
    $this->messenger()
      ->addStatus($this->t('User restriction was created for %label.', ['%label' => $this->entity->label()]));
    // Redirect to the user restriction collection page.
    $form_state->setRedirect('entity.user_restriction.collection');
  }

  /**
   * {@inheritdoc}
   */
  public function actions(array $form, FormStateInterface $form_state): array {
    $actions = parent::actions($form, $form_state);
    $actions['submit']['#value'] = $this->t('Create new user restriction');

    return $actions;
  }

Both the UI strings can be improved.

Proposed resolution

Use User restriction %label has been created. instead of the first string and Add user restriction instead of the second string. The latter string should also be used for the button shown in the list of user restriction entities, which now uses Add rule.