chore: update Linux VHDs to 2023.08.25 (#5113)

Author: CecileRobertMichon

pkg/api/azenvtypes.go

@@ -148,15 +148,15 @@ var (
 		ImageOffer:     "aks",
 		ImageSku:       "aks-engine-ubuntu-1804-202112",
 		ImagePublisher: "microsoft-aks",
-		ImageVersion:   "2023.04.11",
+		ImageVersion:   "2023.08.25",
 	}
 
 	// AKSUbuntu2004OSImageConfig is the AKS Engine image based on Ubuntu 20.04-LTS.
 	AKSUbuntu2004OSImageConfig = AzureOSImageConfig{
 		ImageOffer:     "aks",
 		ImageSku:       "aks-engine-ubuntu-2004-202301",
 		ImagePublisher: "microsoft-aks",
-		ImageVersion:   "2023.04.11",
+		ImageVersion:   "2023.08.25",
 	}
 
 	// AKSWindowsServer2019OSImageConfig is the aks-engine image based on Windows Server 2019