fix: MariaDB 12 uses numeric foreign keys by default

[joshbmarshall]

MariaDB 12.2 (I think also 12.1) if you do not specify a foreign key name, will use numeric strings ("1", "2", "3" etc)

When you go to drop a foreign key, it puts the constraint name as the key in the array. PHP sees it as a numeric string and changes it to an int.

So e.g. the following would previously fail, with an error that $constraint should be a string, int given:

<?php

final class LmsWithdrawalByTeacher extends \Phinx\Migration\AbstractMigration
{
    public function change(): void
    {
        $this->table('s_class_withdrawal')
            ->addColumn('recorded_by_id', 'integer', [
                'null'  => true,
                'after' => 'date',
            ])
            ->addForeignKey('recorded_by_id', 's_teacher')
            ->update();

        $this->table('s_class_withdrawal')
            ->dropForeignKey('recorded_by_id')
            ->update();
    }
}

Should also fix #924