Update VOEvent packet name to contain IGWN instead of LVC

Proposed change "LVC" -> "IGWN". Related issue on py-gcn: https://github.com/nasa-gcn/pygcn/issues/37

This involves updating packet names here: https://git.ligo.org/computing/gracedb/server/-/blob/f9045632831da7afd736081c05afa44bd0faa3e2/gracedb/annotations/voevent_utils.py#L36-42

Updating the IVORN here: https://git.ligo.org/computing/gracedb/server/-/blob/f9045632831da7afd736081c05afa44bd0faa3e2/config/settings/base.py#L235

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