Change lookback days from 30 to 15

Reduced the lookback days for tie-break fairness metric from 30 to 15.

.github/workflows/autoassign.yml

@@ -34,7 +34,7 @@ jobs:
             const REVIEW_POOL = [
               "pussycat0x","ritikchaddha","DhiyaneshGeek"
             ];
-            const LOOKBACK_DAYS = 30; // only used as a tie-break fairness metric
+            const LOOKBACK_DAYS = 15; // only used as a tie-break fairness metric
             // ================================================
 
             function toLowerAll(xs){ return xs.map(x => x.toLowerCase()); }