chore: rev Linux VHDs to 2021.01.08 (#4159)

Author: jackfrancis

pkg/api/azenvtypes.go

@@ -123,15 +123,15 @@ var (
 		ImageOffer:     "aks",
 		ImageSku:       "aks-engine-ubuntu-1604-202007",
 		ImagePublisher: "microsoft-aks",
-		ImageVersion:   "2020.12.02",
+		ImageVersion:   "2021.01.08",
 	}
 
 	// AKSUbuntu1804OSImageConfig is the AKS image based on Ubuntu 18.04-LTS.
 	AKSUbuntu1804OSImageConfig = AzureOSImageConfig{
 		ImageOffer:     "aks",
 		ImageSku:       "aks-engine-ubuntu-1804-202007",
 		ImagePublisher: "microsoft-aks",
-		ImageVersion:   "2020.12.02",
+		ImageVersion:   "2021.01.08",
 	}
 
 	// AKSWindowsServer2019OSImageConfig is the AKS image based on Windows Server 2019