Prolong the expiry milestone for enable-webrtc-pipewire-capturer

The main reason for the flag is so called dialog hell, when you try to
share the screen on Wayland through PipeWire. To address it we've created
two changes, that are currently under review[0][1]. Once that's done the
flag could be removed.

[0] - https://webrtc-review.googlesource.com/c/src/+/160649
[1] - https://chromium-review.googlesource.com/c/chromium/src/+/1942327

Bug: 682122
Change-Id: I2b194f0b1612ca7f9865d0cb4a45f5b5aa5e1c81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1942647
Auto-Submit: Tomáš Popela <tomas.popela@gmail.com>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720476}

chrome/browser/flag-metadata.json

@@ -2124,7 +2124,7 @@
   {
     "name": "enable-webrtc-pipewire-capturer",
     "owners": [ "tomas.popela@gmail.com" ],
-    "expiry_milestone": 79
+    "expiry_milestone": 81
   },
   {
     "name": "enable-webrtc-remote-event-log",