A level 1 wizard casting ray of frost deals 1d4 + their spellcasting modifier in damage.
As you noted, a cantrip is automatically heightened to 1/2 of the wizard's level, rounded up. A level 1 wizard can therefore heighten something to 0.5*1, rounded up. 0.5 * 1 = 1. Therefore, the cantrip can only be cast as a level 1 spell (it's minimum level). As a level 1 spell, ray of frost deals 1d4 damage plus their spellcasting modifier.
A different way to think about this (but which reaches the same conclusion) is that ray of frost is a level 1 cantrip. A cantrip is a spell. Given that the highest level spell slot your level 1 wizard has is also level 1, there is no real possibility of heightening the spell. The cantrips page which you linked to says it this way:
For a typical spellcaster, this means its level is equal to the highest level of spell slot you have.
If a character received basic spellcasting through an archetype, then this may be slightly different. In this case it is possible to have a cantrip heightened to a level beyond the spell slots they have available.
For example, a 4th-level fighter with the basic wizard spellcasting feat has a single level 1 spell slot. However, cantrips are automatically heightened to half their level - making them level 2 ( level 4 / ).