Roll all upstream iOS tests to run on 12a7209.

The xcode_build_version is already 12a7209 but I'm not sure where it is used.
The xcode installed is 12a8189n.
https://ci.chromium.org/p/chromium/builders/webrtc.fyi/WebRTC%20Chromium%20FYI%20ios-device/7690

Bug: 1128632, webrtc:12134
Change-Id: I4403d87f4056524f31b6e19a21e17851aa07eb5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2556944
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830613}

ios/build/bots/chromium.webrtc.fyi/WebRTC Chromium FYI ios-device.json

@@ -3,7 +3,7 @@
     "Builder for 64-bit devices.",
     "Build is performed with gn+ninja."
   ],
-  "xcode build version": "12a8189n",
+  "xcode build version": "12a7209",
   "gn_args": [
     "goma_dir=\"$(goma_dir)\"",
     "ios_enable_code_signing=false",

ios/build/bots/chromium.webrtc.fyi/WebRTC Chromium FYI ios-simulator.json

@@ -2,7 +2,7 @@
   "comments": [
     "Runs tests on @3x, @2x, 64-bit phone, tablet, iOS 12."
   ],
-  "xcode build version": "12a8189n",
+  "xcode build version": "12a7209",
   "gn_args": [
     "goma_dir=\"$(goma_dir)\"",
     "ios_enable_code_signing=false",