Sync old season before beginning new season

[EthanThatOneKid]

Background

#53 describes a bug in which the final day is not being properly recorded before beginning the new season.

Intends to resolve #53.

Relevant code

lc-dailies/lib/leaderboard/denokv/denokv_leaderboard_client.ts

Lines 160 to 175 in 590559c

	if (!isPresentAndLatest && seasonResult?.value) {
	// Sync old season.
	const players = await this.listPlayers();
	const oldSeason = await sync({
	lcClient: this.lc,
	players,
	season: seasonResult.value,
	});
	oldSeason.synced_at = referenceDate.toUTCString();
	// Store the synced old season.
	await this.kv.set(
	[LeaderboardKvPrefix.SEASONS, oldSeason.id],
	oldSeason,
	);
	}

[tomasohCHOM]

I don't know if I am the most suitable person to approve this change, but so far it looks like the logic plays out, LGTM!