Before this commit, the monthly dossiers count was serialized into the
Stat record using human-formatted dates, as:
```ruby
s.dossiers_in_the_last_4_months = {
"octobre 2021"=>409592,
"novembre 2021"=>497823,
"décembre 2021"=>38170,
"janvier 2022"=>0
}
```
Turns out the ordering of keys in a serialized hash is not guaranteed.
After a round-trip to the database, the keys will be wrongly sorted.
Instead we want to save raw Date objects, which will preserve the
ordering. The date formatting can be applied at display-time by the
controller.
Fix#6848
e6cf07b810