Ensure earliest allowed block is at minimum the next block (#4823)

When `min_enactment_period == 0` and `desired == At(n)` where `n` is
smaller than the current block number, the scheduling would fail. This
happened for example here:
https://collectives.subsquare.io/fellowship/referenda/126

To ensure that this doesn't happen again, ensure that the earliest
allowed block is at minimum the next block.