Change expiration milestone for iOS text zoom flags

The text zoom experiment is going on longer than expected, so the flags
are still necessary.

Fixed: 1159090, 1143362, 1143361
Change-Id: Ibd5f7aed10b774218ee1aebe7e010c7ece5cf2ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2598847Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/master@{#840143}

chrome/browser/flag-metadata.json

@@ -5024,17 +5024,17 @@
   {
     "name": "webpage-alternative-text-zoom",
     "owners": [ "rkgibson@google.com", "bling-flags@google.com" ],
-    "expiry_milestone": 86
+    "expiry_milestone": 91
   },
   {
     "name": "webpage-default-zoom-from-dynamic-type",
     "owners": [ "rkgibson@google.com", "bling-flags@google.com" ],
-    "expiry_milestone": 87
+    "expiry_milestone": 91
   },
   {
     "name": "webpage-text-accessibility",
     "owners": [ "rkgibson@google.com", "bling-flags@google.com" ],
-    "expiry_milestone": 86
+    "expiry_milestone": 91
   },
   {
     "name": "webui-dark-mode",