Use uuid version 6

[Markus Reinhold]

Although not accepted as a standard, the community has agreed on one. With uuid version 6, the bit shuffling of version 1 is no longer needed, since the time is represented in the first bits. Both, ramsey/uuid and symfony/uid can generate this version. To keep the number of vendors low, we change to symfony/uid as the doctrine type uuid_binary_ordered_time isn't needed anymore. This applies for all contexts.