fix(naming): use same name in ws and batch for exposedButNotAtRiskUsers

Mismatch in kpi naming between :

• https://gitlab.inria.fr/stopcovid19/robert-server/-/blob/develop/robert-server-ws-rest/src/main/kotlin/fr/gouv/stopc/robertserver/ws/repository/model/Kpi.kt#L21
• https://gitlab.inria.fr/stopcovid19/robert-server/-/blob/develop/robert-server-batch/src/main/java/fr/gouv/stopc/robert/server/batch/tasklet/SaveKpisTasklet.java#L31