Make Linux bots run on an unspecified Ubuntu version

Previously they would only run on Ubuntu 14.04 which has been removed
from the luci flex pool. Run on unspecified Ubuntu version so the
tryjobs start working again.

TBR=enga@chromium.org
BUG=

Change-Id: I32d584ae6ef90a5add926c3c6c60a1d45aed0d45
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/8441
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

infra/config/global/cr-buildbucket.cfg

diff --git a/infra/config/global/cr-buildbucket.cfg b/infra/config/global/cr-buildbucket.cfg
index 6dca5f6..88b541b 100644
--- a/infra/config/global/cr-buildbucket.cfg
+++ b/infra/config/global/cr-buildbucket.cfg
@@ -104,25 +104,25 @@
     # Linux: test combinations of {clang}x{release,debug}x{x86,x64}
     builders {
       name: "linux-clang-dbg-x64"
-      dimensions: "os:Ubuntu-14.04"
+      dimensions: "os:Ubuntu"
       mixins: "debug"
       mixins: "x64"
     }
     builders {
       name: "linux-clang-dbg-x86"
-      dimensions: "os:Ubuntu-14.04"
+      dimensions: "os:Ubuntu"
       mixins: "debug"
       mixins: "x86"
     }
     builders {
       name: "linux-clang-rel-x64"
-      dimensions: "os:Ubuntu-14.04"
+      dimensions: "os:Ubuntu"
       mixins: "release"
       mixins: "x64"
     }
     builders {
       name: "linux-clang-rel-x86"
-      dimensions: "os:Ubuntu-14.04"
+      dimensions: "os:Ubuntu"
       mixins: "release"
       mixins: "x86"
     }
@@ -212,7 +212,7 @@
 
     builders {
       name: "presubmit"
-      dimensions: "os:Ubuntu-14.04"
+      dimensions: "os:Ubuntu"
       recipe {
         name: "run_presubmit"
         properties: "repo_name:dawn"
@@ -227,28 +227,28 @@
     # CI Mirror only
     builders {
       name: "linux-clang-dbg-x64"
-      dimensions: "os:Ubuntu-14.04"
+      dimensions: "os:Ubuntu"
       mixins: "debug"
       mixins: "x64"
     }
     # Actually on the CQ
     builders {
       name: "linux-clang-dbg-x86"
-      dimensions: "os:Ubuntu-14.04"
+      dimensions: "os:Ubuntu"
       mixins: "debug"
       mixins: "x86"
     }
     # Actually on the CQ
     builders {
       name: "linux-clang-rel-x64"
-      dimensions: "os:Ubuntu-14.04"
+      dimensions: "os:Ubuntu"
       mixins: "release"
       mixins: "x64"
     }
     # Actually on the CQ
     builders {
       name: "linux-clang-rel-x86"
-      dimensions: "os:Ubuntu-14.04"
+      dimensions: "os:Ubuntu"
       mixins: "release"
       mixins: "x86"
     }