Revert the UbuntuLTS image alias to use the version 16.04 instead of 20.04 (#20792)

* Update Images.json

* Update ChangeLog.md

---------

Co-authored-by: Yeming Liu <[email protected]>

Author: Sandido

src/Compute/Compute/ChangeLog.md

@@ -25,6 +25,7 @@
 * Filled in missing parameter descriptions across multiple parameters and improved some existing parameter descriptions.
 * Updated Compute PS to use the new .Net SDK version 59.0.0. This includes an approved breaking change for a non-functional feature. 
   - The type of the property `Source` of type `Microsoft.Azure.Management.Compute.Models.GalleryDataDiskImage`, `Microsoft.Azure.Management.Compute.Models.GalleryOSDiskImage`, and `Microsoft.Azure.Management.Compute.Models.GalleryImageVersionStorageProfile` has changed from `Microsoft.Azure.Management.Compute.Models.GalleryArtifactVersionSource` to `Microsoft.Azure.Management.Compute.Models.GalleryDiskImageSource`.
+* Updated the broken `UbuntuLTS` image alias to use its original sku version of `16.04-LTS` instead of the nonexistent image `20.04-LTS`. This fixes an issue introduced in the version 5.3.0 release. 
 * Updated Set-AzVMRunCommand and Set-AzVmssRunCommand ScriptLocalPath parameter set to work with Linux and with files that have comments.
 * Added `-TargetExtendedLocation` parameter to `New-AzGalleryImageVersion` and `Update-AzGalleryImageVersion`
 * Added `-AllowDeletionOfReplicatedLocation` to `Update-AzGalleryImageVersion`

src/Compute/Compute/Strategies/ComputeRp/Images.json

@@ -39,7 +39,7 @@
                   "UbuntuLTS":  {
                                     "publisher":  "Canonical",
                                     "offer":  "UbuntuServer",
-                                    "sku":  "20.04-LTS",
+                                    "sku":  "16.04-LTS",
                                     "version":  "latest"
                                 }
               },