[6.x] Fix error when publishing webauthn migration

[duncanmcclean]

This pull request fixes an error when the update script attempts to publish the webauthn migration:

Running update script [Statamic\UpdateScripts\PublishMigrationForWebauthnTable]

   Illuminate\Contracts\Filesystem\FileNotFoundException

  File does not exist at path <?php

use Illuminate\Support\Facades\Schema;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Database\Migrations\Migration;

class StatamicWebauthnTable extends Migration
{

Caused by #13769 (sorry! 😬)