Filter out Bursts from SubGRB search

A bug was found where we are doing an unintended a SubGRB-Burst when triggering off a external event, where if triggering off a SubGRB we still listen to group='Burst' events: https://git.ligo.org/emfollow/gwcelery/-/blob/48d38582049de225eb5ba4002747f9dfd375666e/gwcelery/tasks/external_triggers.py#L253-259

A group='CBC' and CWB BBH searches need to be added here.

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