The under parameter in the spreadplayers command does not support relative coordinates

This issue is related to MC-233599. In the 1.16 update, a new "under" parameter was added to the /spreadplayers command, allowing for the specification of the maximum Y-coordinate for entity teleportation. However, unlike the "center" parameter or other cases where coordinates are allowed, this argument only accepts absolute coordinates, not relative ones. Although this issue may be viewed as a feature request, it's being reported nonetheless. This issue restricts the versatility of the /spreadplayers command.

Steps to reproduce

1. /spreadplayers ~ ~ 0 1 under 100 false @s

2. /spreadplayers ~ ~ 0 1 under ~ false @s

3. /spreadplayers ~ ~ 0 8 under ~8 false @s

Observed result: The command functions only with an absolute coordinate input, not with a relative Y-coordinate.
Expected result: The command should also function with relative Y-coordinates.