goma/server: use Ubuntu-18.04

Change-Id: I60c2881760da376e302e760687615019e3354b1a
diff --git a/infra/config/generated/cr-buildbucket.cfg b/infra/config/generated/cr-buildbucket.cfg
index f74bbfe..ff50a43 100644
--- a/infra/config/generated/cr-buildbucket.cfg
+++ b/infra/config/generated/cr-buildbucket.cfg
@@ -18,7 +18,7 @@
       name: "linux_rel"
       swarming_host: "chromium-swarm.appspot.com"
       dimensions: "cpu:x86-64"
-      dimensions: "os:Ubuntu-16.04"
+      dimensions: "os:Ubuntu-18.04"
       dimensions: "pool:luci.flex.try"
       recipe {
         name: "goma_server"
diff --git a/infra/config/main.star b/infra/config/main.star
index 907e903..2133e86 100755
--- a/infra/config/main.star
+++ b/infra/config/main.star
@@ -92,7 +92,7 @@
     ),
     dimensions = {
         "pool": "luci.flex.try",
-        "os": "Ubuntu-16.04",
+        "os": "Ubuntu-18.04",
         "cpu": "x86-64",
     },
 )