http2: replace var with let/const

PR-URL: nodejs#30417
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

lib/internal/http2/util.js

@@ -197,7 +197,7 @@ const IDX_OPTIONS_MAX_SESSION_MEMORY = 8;
 const IDX_OPTIONS_FLAGS = 9;
 
 function updateOptionsBuffer(options) {
-  var flags = 0;
+  let flags = 0;
   if (typeof options.maxDeflateDynamicTableSize === 'number') {
     flags |= (1 << IDX_OPTIONS_MAX_DEFLATE_DYNAMIC_TABLE_SIZE);
     optionsBuffer[IDX_OPTIONS_MAX_DEFLATE_DYNAMIC_TABLE_SIZE] =
@@ -318,7 +318,7 @@ function getSettings(session, remote) {
 }
 
 function updateSettingsBuffer(settings) {
-  var flags = 0;
+  let flags = 0;
   if (typeof settings.headerTableSize === 'number') {
     flags |= (1 << IDX_SETTINGS_HEADER_TABLE_SIZE);
     settingsBuffer[IDX_SETTINGS_HEADER_TABLE_SIZE] =
@@ -527,11 +527,11 @@ const assertWithinRange = hideStackFrames(
 function toHeaderObject(headers) {
   const obj = Object.create(null);
   for (var n = 0; n < headers.length; n = n + 2) {
-    var name = headers[n];
-    var value = headers[n + 1];
+    const name = headers[n];
+    let value = headers[n + 1];
     if (name === HTTP2_HEADER_STATUS)
       value |= 0;
-    var existing = obj[name];
+    const existing = obj[name];
     if (existing === undefined) {
       obj[name] = name === HTTP2_HEADER_SET_COOKIE ? [value] : value;
     } else if (!kSingleValueHeaders.has(name)) {