[6.x] Fix whereDate() and whereTime() when the app timezone is not UTC

[lazerg]

The Stache query builder never matches anything with whereDate() or whereTime() when app.timezone is set to something other than UTC. A knock-on effect is that scheduled entries stop getting published, since MinuteEntries is built on both of those methods.

The comparison value is normalised into the app timezone in Statamic\Query\Builder, but the indexed values are augmented dates and therefore in UTC. filterWhereDate() then calls startOfDay() on the UTC value, which lands on midnight UTC, while the comparison value sits at midnight in the app timezone. Those are two different instants, so with any non-zero offset the two sides can never line up. filterWhereTime() has the same problem: it applies the app timezone H:i:s string to a UTC value.

Converting the indexed value into the app timezone before comparing puts both sides on the same wall clock. Statamic\Query\IteratorBuilder carries a copy of the same logic, so search and item queries were affected too and are fixed alongside it.

Fixes #15336