Migrate linux CI builder to bionic from xenial

https://luci-milo.appspot.com/p/gn/builders/ci/linux

The infra fleet is moving from xenial to bionic. This bot needs
to move with it.

Bug: chromium:1199405
Change-Id: I455f82cba5a4102982c2f22cb88c3983d223cd90
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/14020
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>

infra/config/generated/cr-buildbucket.cfg

diff --git a/infra/config/generated/cr-buildbucket.cfg b/infra/config/generated/cr-buildbucket.cfg
index 663243a..9815e2b 100644
--- a/infra/config/generated/cr-buildbucket.cfg
+++ b/infra/config/generated/cr-buildbucket.cfg
@@ -14,7 +14,7 @@
       name: "linux"
       swarming_host: "chromium-swarm.appspot.com"
       dimensions: "cpu:x86-64"
-      dimensions: "os:Ubuntu-16.04"
+      dimensions: "os:Ubuntu-18.04"
       dimensions: "pool:luci.flex.ci"
       recipe {
         name: "gn"

infra/config/main.star

diff --git a/infra/config/main.star b/infra/config/main.star
index 876d20b..1357967 100755
--- a/infra/config/main.star
+++ b/infra/config/main.star
@@ -97,7 +97,7 @@
         short_name = name,
     )
 
-ci_builder("linux", "Ubuntu-16.04")
+ci_builder("linux", "Ubuntu-18.04")
 ci_builder("mac", "Mac-10.15", caches = [swarming.cache("macos_sdk")])
 ci_builder("win", "Windows-10", caches = [swarming.cache("windows_sdk")])