Rename LVC to IGWN in VOEvent channel name and IVORN
This change should involve updating the packet names here: https://git.ligo.org/computing/gracedb/server/-/blob/f9045632831da7afd736081c05afa44bd0faa3e2/gracedb/annotations/voevent_utils.py#L36-42
And the IVORN here: https://git.ligo.org/computing/gracedb/server/-/blob/f9045632831da7afd736081c05afa44bd0faa3e2/config/settings/base.py#L235
Need to coordinate with related py-gcn issue: https://github.com/nasa-gcn/pygcn/issues/37