Lower the Postgres default disk size to 1 GB (#3481)

A 10 GB default disk size hinders `fly launch` by requiring a
credit card, which introduces friction for anyone just interested
in trying us out.

More here: https://flyio.discourse.team/t/making-the-rails-speedrun-more-credible/5512/18?u=clouvet

internal/command/launch/plan/postgres.go

@@ -34,7 +34,7 @@ func DefaultPostgres(plan *LaunchPlan) PostgresPlan {
 			VmSize:     "shared-cpu-1x",
 			VmRam:      1024,
 			Nodes:      1,
-			DiskSizeGB: 10,
+			DiskSizeGB: 1,
 		},
 	}
 }