Ubuntu 20.04 instead of 18.04 (#15525)

net/net.sh

@@ -168,7 +168,7 @@ annotateBlockexplorerUrl() {
 }
 
 build() {
-  supported=("18.04")
+  supported=("20.04")
   declare MAYBE_DOCKER=
   if [[ $(uname) != Linux || ! " ${supported[*]} " =~ $(lsb_release -sr) ]]; then
     # shellcheck source=ci/rust-version.sh

net/scripts/gce-provider.sh

@@ -172,10 +172,10 @@ cloud_CreateInstances() {
   #declare sshPrivateKey="${13}"  # unused
 
   if $enableGpu; then
-    # Custom Ubuntu 18.04 LTS image with CUDA 9.2 and CUDA 10.0 installed
+    # Custom Ubuntu 20.04 LTS image with CUDA 10.2 installed
     #
     # Unfortunately this image is not public.  When this becomes an issue, use
-    # the stock Ubuntu 18.04 image and programmatically install CUDA after the
+    # the stock Ubuntu 20.04 image and programmatically install CUDA after the
     # instance boots
     #
     imageName="ubuntu-2004-focal-v20201211-with-cuda-10-2 --image-project principal-lane-200702"