`isCurrentRound` is never set to `false` for previous Rounds in subgraph

[isCurrentRound](https://github.com/kleros/kleros-v2/blob/1ca90667db3ca3d5954045edb251a04b452fbb57/subgraph/core/src/entities/Round.ts#L14) is always `true`  regardless if the round is old or current.
