[CrOS Cellular] Extend cellular-forbid-attach-apn flag.

Since Attach APN is still in the process of gradually rolling out via
Finch, we need to keep the infrastructure to kill switch the feature if
critical issues are discovered.

Bug: 1264096
Change-Id: I342dde0d62b1b07a862caa094dda06230ef7f630
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3248762
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#936163}
diff --git a/chrome/browser/flag-metadata.json b/chrome/browser/flag-metadata.json
index e858230..fac587b 100644
--- a/chrome/browser/flag-metadata.json
+++ b/chrome/browser/flag-metadata.json
@@ -651,7 +651,7 @@
   {
     "name": "cellular-forbid-attach-apn",
     "owners": [ "hsuregan", "cros-connectivity@google.com" ],
-    "expiry_milestone": 95
+    "expiry_milestone": 100
   },
   {
     "name": "cellular-use-attach-apn",