lib: use let instead of var

this commit uses let instead of var for util.js & warning.js
PR-URL: #30375
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/internal/process/warning.js

@@ -58,7 +58,7 @@ function onWarning(warning) {
   if (isDeprecation && process.noDeprecation) return;
   const trace = process.traceProcessWarnings ||
                 (isDeprecation && process.traceDeprecation);
-  var msg = `(${process.release.name}:${process.pid}) `;
+  let msg = `(${process.release.name}:${process.pid}) `;
   if (warning.code)
     msg += `[${warning.code}] `;
   if (trace && warning.stack) {

lib/internal/util.js

@@ -166,7 +166,7 @@ function emitExperimentalWarning(feature) {
 
 function filterDuplicateStrings(items, low) {
   const map = new Map();
-  for (var i = 0; i < items.length; i++) {
+  for (let i = 0; i < items.length; i++) {
     const item = items[i];
     const key = item.toLowerCase();
     if (low) {
@@ -281,7 +281,7 @@ function promisify(original) {
         }
         if (argumentNames !== undefined && values.length > 1) {
           const obj = {};
-          for (var i = 0; i < argumentNames.length; i++)
+          for (let i = 0; i < argumentNames.length; i++)
             obj[argumentNames[i]] = values[i];
           resolve(obj);
         } else {