Fix: schemas: Add missing valid date expr rule options

According to pe__eval_date_expr(), in_range accepts a start, an end, or
both, along with an optional duration. If an end and duration are both
specified, the duration is simply ignored.

There was also an unnecessary <choice> surrounding a single option for
lt.

Ref T196

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>