PG: set info.last_epoch_started on activate even if backfill

If we only find backfill peers, we still want min_last_update_acceptable
to be determined by those backfill peers.  I think this was an oversight
from the acting->actingbackfill switch.

Fixes: #9482
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>