Unknown database type json requested, Doctrine\DBAL\Platforms\MySqlPlatform may not support it.

# 场景：迁移文件 修改表字段：由 json 改为 string 时出现
# 解决办法：迁移文件中添加如下代码

    public function up()
    {
        DB::getDoctrineSchemaManager()->getDatabasePlatform()->registerDoctrineTypeMapping('json', 'string');
        Schema::table('table_name',function (Blueprint $blueprint){
           $blueprint->string('field_name')->change();
        });
    }