http: replace var with let

PR-URL: #30421
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: Guilherme Goncalves

lib/internal/http.js

@@ -3,8 +3,8 @@
 const { setUnrefTimeout } = require('internal/timers');
 const { PerformanceEntry, notify } = internalBinding('performance');
 
-var nowCache;
-var utcCache;
+let nowCache;
+let utcCache;
 
 function nowDate() {
   if (!nowCache) cache();