Fix get_astral_location deprecation warning

[dmamelin]

sunrise/sunset triggers used sun.get_astral_location(), which HA deprecated (removal in 2027.7) and warned on every use. Switched to the higher-level sun.get_astral_event_date(), which avoids the deprecated API entirely and needs no version fallback.

This also restores elevation support, which was inadvertently dropped in 300a633 when a local get_astral_event_date copy omitted the observer_elevation argument. No behavior change at the default elevation of 0; existing tests pass.

On dropping async_add_executor_job: the executor wrapping was added in 884c428 (PR #610) to move the blocking location.sunrise()/sunset() calls (timezone data loaded from disk) off the event loop. That's no longer needed here: get_astral_event_date() is a HA @callback (loop-safe by contract, and called synchronously by HA itself), and it computes in UTC - the local conversion uses the timezone already cached at startup via dt_util.as_local(). So the call runs directly on the loop without reintroducing the blocking I/O.

Fixes #858.

[craigbarratt]

Thanks for the PR; much cleaner now!