Fix: Update isCurrentRound for previous round on appeal

The isCurrentRound field in the Round entity was not being set to false for the previous round when a new round was created due to an appeal.

This commit modifies the handleAppealDecision handler in KlerosCore.ts to:
1. Load the round that was current before the appeal.
2. Set its isCurrentRound field to false and save it.
3. Proceed to create the new round, which will have isCurrentRound set to true by the createRoundFromRoundInfo function.

Newly added test files (kleros-core.test.ts and kleros-core-utils.ts) were removed from this commit due to feedback regarding outdated testing tooling. The core logic fix remains.

Author: google-labs-jules[bot]