Revert "refactor: 可読性のため一部でArray.prototype.atを使うように" (#115)

* Revert "refactor: 可読性のため一部で`Array.prototype.at`を使うように (#11274)"

This reverts commit 2b6dbd4fcb.

* Apply suggestions from code review

---------

Co-authored-by: okayurisotto <okayurisotto@proton.me>

packages/backend/src/queue/processors/ExportMutingProcessorService.ts

@@ -76,7 +76,7 @@ export class ExportMutingProcessorService {
 					break;
 				}
 
-				cursor = mutes.at(-1)?.id ?? null;
+				cursor = mutes[mutes.length - 1].id;
 
 				for (const mute of mutes) {
 					const u = await this.usersRepository.findOneBy({ id: mute.muteeId });