Fix constant when extracting timestamp from UUIDv7.

Fix constant when extracting timestamp from UUIDv7.

When extracting a timestamp from a UUIDv7, a conversion from
milliseconds to microseconds was using the incorrect constant
NS_PER_US instead of US_PER_MS. Although both constants have the same
value, this fix improves code clarity by using the semantically
correct constant.

Backpatch to v18, where UUIDv7 was introduced.

Author: Erik Nordström <[email protected]>
Reviewed-by: Andrey Borodin <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Discussion: https://postgr.es/m/CACAa4V+i07eaP6h4MHNydZeX47kkLPwAg0sqe67R=M5tLdxNuQ@mail.gmail.com
Backpatch-through: 18