Opt out of C++23 when building in Chromium

See https://groups.google.com/a/google.com/g/chromium-dev-internal/c/22inHeL2Di8/m/P929_yUSEgAJ
for the announcement. Some of Skia's clients are C++20 only (e.g.
GCC 10.x support) for now.

Change-Id: I0f857e5575d042daef8a7e1a262934d18ee65f7d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1140758
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

.gn

diff --git a/.gn b/.gn
index 5ef782b..5b619b23 100644
--- a/.gn
+++ b/.gn
@@ -7,4 +7,7 @@
   angle_use_wayland = false
   dawn_use_built_dxc = false
   dawn_force_system_component_load = true
+
+  # Skia must stay in C++20 for longer compared to Chromium.
+  use_cxx23 = false
 }