Merge cockroachdb#59361

59361: roachprod: replace guidance on more modern GCE image r=ajwerner a=ajwerner

Using the old recommended image results in the following error:

```
Output: ERROR: (gcloud.compute.instances.create) Could not fetch resource:
 - The resource 'projects/ubuntu-os-cloud/global/images/ubuntu-1904-disco-v20191008' is obsolete.  New uses are not allowed.  No replacement was specified.: exit status 1)
```

Release note: None

Co-authored-by: Andrew Werner <ajwerner@cockroachlabs.com>

pkg/cmd/roachprod/vm/gce/gcloud.go

@@ -274,7 +274,7 @@ func (o *providerOpts) ConfigureCreateFlags(flags *pflag.FlagSet) {
 	flags.StringVar(&o.MinCPUPlatform, ProviderName+"-min-cpu-platform", "",
 		"Minimum CPU platform (see https://cloud.google.com/compute/docs/instances/specify-min-cpu-platform)")
 	flags.StringVar(&o.Image, ProviderName+"-image", "ubuntu-1604-xenial-v20200129",
-		"Image to use to create the vm, ubuntu-1904-disco-v20191008 is a more modern image")
+		"Image to use to create the vm, ubuntu-2004-focal-v20210119a is a more modern image")
 
 	flags.IntVar(&o.SSDCount, ProviderName+"-local-ssd-count", 1,
 		"Number of local SSDs to create, only used if local-ssd=true")