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: I2b0eba4a992c992064f52554012d8913bb2706fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2106160
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@{#750936}

chrome/browser/flag-metadata.json

@@ -2199,7 +2199,7 @@
   {
     "name": "enable-webrtc-pipewire-capturer",
     "owners": [ "tomas.popela@gmail.com" ],
-    "expiry_milestone": 82
+    "expiry_milestone": 83
   },
   {
     "name": "enable-webrtc-remote-event-log",