DOC: Update the stale reindex note in ARCHModel.forecast

[VenishPaneliya]

ARCHModel.forecast's reindex entry still describes the 4.19 transition:

Prior to 4.18 this was the default. As of 4.19 this is now optional. If not provided, a warning is raised about the future change in the default which will occur after September 2021.

No warning is raised any more, and the default is already False:

reindex default                        : False
warnings mentioning reindex when omitted: NONE
default (reindex omitted) variance rows : 1
reindex=True              variance rows : 400     # len(y) == 400

ARCHModelFixedResult.forecast carries the same entry and was updated in 6.2, so I copied its wording verbatim; the two docstrings are otherwise byte-identical. "September 2021" now appears nowhere in the tree. Docs only.

[bashtage]

Thanks.