Daml: Consistent enforcement of `requestedAt` times

https://github.com/canton-network/splice/pull/7839#discussion_r4322983501

what's the logic for enforcing that it's not older than 24h? iirc in all existing uses where we specify a requestedAt field we just check that it's in the past which seems all that's required for correctness. Not against fat finger protection but then it seems like it should be consistently applied.

it's fat finger protection. it's in particular important for checking that an unlock time is in the close future. We then applied it consistently for this work.