update azure image placeholder (#5182)

Author: skanakal

translations/en-us.yaml

@@ -9458,7 +9458,7 @@ nodeDriver:
       placeholder: Your Azure subscription certificate
     image:
       label: Image
-      placeholder: 'canonical:UbuntuServer:18.04-LTS:latest'
+      placeholder: 'canonical:ubuntu-24_04-lts:server-gen1:latest'
       helpText: Providing an ARM resource identifier requires using managed disk
     diskSize:
       label: Disk Size