Extend expiry of SignedExchange related flags

- allow-sxg-certs-without-extension is still used by developers for
  testing.
- enable-sxg-prefetch-cache-for-navigations and
  enable-sxg-subresource-prefetching are related to SignedExchange
  Subresource prefetching, which is shipped in September. Being able to
  disable it with a flag would be useful for diagnosis, so let's keep
  them for a while.

Bug: 862003,935267,968427
Change-Id: Ice8942664b5418a1c0119f65d89eed1fa507a21f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2509153Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823075}

chrome/browser/flag-metadata.json

@@ -90,7 +90,7 @@
     "owners": [ "//content/browser/web_package/OWNERS" ],
     // Used by developers for testing signed exchange loading using normal
     // certs. See https://crbug.com/862003
-    "expiry_milestone": 87
+    "expiry_milestone": 92
   },
   {
     "name": "allow-sync-xhr-in-page-dismissal",
@@ -2156,12 +2156,12 @@
   {
     "name": "enable-sxg-prefetch-cache-for-navigations",
     "owners": [ "horo", "//content/browser/web_package/OWNERS" ],
-    "expiry_milestone": 87
+    "expiry_milestone": 92
   },
   {
     "name": "enable-sxg-subresource-prefetching",
     "owners": [ "horo", "//content/browser/web_package/OWNERS" ],
-    "expiry_milestone": 87
+    "expiry_milestone": 92
   },
   {
     "name": "enable-tab-engagement-reporting",