Fix TimeSensor start_from_trigger causing dag_version churn

[bramhanandlingala]

closes: #69543

TimeSensor computed its target time using datetime.now() at DAG-parse
time. When start_from_trigger=True, this value got baked into the
serialized DAG, causing the dag_version to change on every parse that
crossed a day boundary.

This can't be fixed by computing the value differently, since the target
moment is inherently different each day but start_trigger_args is only
computed once per DAG version. So start_from_trigger=True now raises a
ValueError instead of silently producing a broken serialization.
deferrable=True still works as before and is unaffected.

Also fixed the docs, which incorrectly said the target time is evaluated
against data_interval_end/run_after — it never was.

Added a test for the new ValueError; all existing tests still pass.