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/ExportNotesProcessorService.ts

@@ -94,7 +94,7 @@ export class ExportNotesProcessorService {
 					break;
 				}
 
-				cursor = notes.at(-1)?.id ?? null;
+				cursor = notes[notes.length - 1].id;
 
 				for (const note of notes) {
 					let poll: Poll | undefined;