message: Fix era cleanup.
  The code accidentally cleaned up the current era instead of the old one.
diff: @@ -370,7 +370,7 @@ where
  // to for cross-era slashing.
  if let Some(evidence_only_era_id) = era_id.checked_sub(self.bonded_eras + 1) {
  trace!(era = evidence_only_era_id.0, "clearing unbonded era");
  - if let Some(era) = self.active_eras.get_mut(&era_id) {
  + if let Some(era) = self.active_eras.get_mut(&evidence_only_era_id) {
  era.consensus.set_evidence_only();
  }
  }
  