Fix for typo in UUIDv7 timestamp extraction

Hi,

I think I found a small typo in the function that extracts a timestamp from
a UUIDv7 (uuid_extract_timestamp). Unless I am mistaken, the constant
US_PER_MS should be used instead of NS_PER_US when converting milliseconds
to microseconds. Fortunately, these constants are the same so the
calculation is still correct.

Anyway, attaching a patch to fix this typo.

On a related note, I am wondering why this function doesn't extract and use
the sub-millisecond information in the rand_a bits? These bits are added
when generating the UUID, but they don't seem to be extracted. Hopefully
somebody could shed some light on this and whether it would be a worthwhile
addition.

Thank you,

- Erik