Remove gallery preview version 2020-09-30 from stable release (#12222)

Author: Sandido

specification/compute/resource-manager/readme.md

@@ -219,8 +219,7 @@ input-file:
 - Microsoft.Compute/stable/2020-06-01/runCommands.json
 - Microsoft.Compute/stable/2019-04-01/skus.json
 - Microsoft.Compute/stable/2020-06-30/disk.json
-- Microsoft.Compute/preview/2020-09-30/gallery.json
-- Microsoft.Compute/preview/2020-09-30/sharedGallery.json
+- Microsoft.Compute/stable/2019-12-01/gallery.json
 - Microsoft.ContainerService/stable/2017-01-31/containerService.json
 - Microsoft.Compute/preview/2020-10-01-preview/cloudService.json
 ```
@@ -235,7 +234,6 @@ input-file:
 ```
 
 ### Tag: package-2020-09-30
-
 These settings apply only when `--tag=package-2020-09-30` is specified on the command line.
 
 ``` yaml $(tag) == 'package-2020-09-30'
@@ -810,4 +808,4 @@ input-file:
 
 ## AzureResourceSchema
 
-See configuration in [readme.azureresourceschema.md](./readme.azureresourceschema.md)
+See configuration in [readme.azureresourceschema.md](./readme.azureresourceschema.md)