Dates array should be sorted chronologically

[chris48s]

Problem: At the moment, the dates[] array is just sorted in the order we add the ballots in.

This means that we can do something like

dc_response_builder/response_builder/v1/generated_responses/root_responses.py

Lines 68 to 78 in e22b430

	MULTIPLE_BALLOTS_WITH_VOTING_SYSTEM_AND_POLLING_STATION = (
	RootBuilder()
	.with_multiple_ballots(
	[
	MAYOR_BALLOT.with_candidates(candidates.all_candidates).build(),
	GLA_BALLOT.with_candidates(candidates.all_candidates).build(),
	]
	)
	.with_polling_station(WITHOUT_MAP_POLLING_STATION.build())
	.with_electoral_services(electoral_services.stroud_electoral_services)
	)

In this example, the GLA_BALLOT has a date before the MAYOR_BALLOT

dc_response_builder/response_builder/v1/generated_responses/ballots.py

Lines 18 to 25 in e22b430

	GLA_BALLOT = (
	StockLocalBallotBuilder()
	.with_ballot_title("London Assembly elections")
	.with_post_name("London Assembly")
	.with_election_name("London Assembly elections")
	.with_ballot_paper_id("gla.a.2024-05-02")
	.with_election_id("gla.a.2024-05-02")
	)

dc_response_builder/response_builder/v1/generated_responses/ballots.py

Lines 40 to 49 in e22b430

	MAYOR_BALLOT = (
	StockLocalBallotBuilder()
	.with_ballot_title("Mayor of London")
	.with_post_name("Mayor of London")
	.with_election_name("Mayor of London")
	.with_ballot_paper_id("mayor.london.2024-11-02")
	.with_election_id("mayor.london.2024-11-02")
	.with_voting_system("FPTP")
	.with_candidates(candidates.all_candidates)
	)

but because we add it second, it comes after it in the dates array. This is unexpected by client apps, the set_date_baseline() function, etc.

Solution: We should ensure that whatever order we add ballots in (either using .with_multiple_ballots() or chained calls to with_ballot() the ballots come out in chronological order in the final response.