Fix change_significance_statement

change_significance_statement determines where the GW is considered sub-threshold and the text is changed in the joint circular. The label SIGNIF_LOCKED was thought to only be applied for threshold GW alerts but must have been applied due to RAVEN_ALERT for https://gracedb.ligo.org/superevents/S250223dk/view/:

https://git.ligo.org/emfollow/ligo-followup-advocate/-/blob/c832cc1f3d1141315eed811484126cd376126878/ligo/followup_advocate/templates/initial_body.jinja2#L15

We should think harder about what these labels should be in the future to prevent this from occurring again.