* Default index mode null for TimeSeries

If the default index mode matches the specified, we skip printing the
synthetic source info in the mappings printing. This leads to confusion
as it's not immediately visible (or well known) that time series indices
use synthetic source by default.

Leaving the default index mode to null does the trick here. We do pass
the right value for time series indexes while building the mapping so
there's no functional impact here.

Fixes elastic#97429

* Update docs/changelog/98586.yaml

* Restore other error messages.

* Update source in DocumentMapper to include synthetic source.

* Add version check for skipping assert