[ReleasePR sql] Resume V2021-02-01-Preview BackupShortTermRetentionPolicies API Specification (#1763)

Create to sync Azure/azure-rest-api-specs#14671
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/master...AzureSDKAutomation:sdkAuto/sql?expand=1)

Author: msftbot[bot]

schemas/2020-11-01-preview/Microsoft.Sql.json

@@ -1381,9 +1381,6 @@
           "type": "array",
           "items": {
             "oneOf": [
-              {
-                "$ref": "#/definitions/servers_databases_backupShortTermRetentionPolicies_childResource"
-              },
               {
                 "$ref": "#/definitions/servers_databases_extendedAuditingSettings_childResource"
               },
@@ -1408,6 +1405,9 @@
               {
                 "$ref": "#/definitions/servers_databases_workloadGroups_childResource"
               },
+              {
+                "$ref": "#/definitions/servers_databases_backupShortTermRetentionPolicies_childResource"
+              },
               {
                 "$ref": "#/definitions/servers_databases_extensions_childResource"
               },

schemas/2021-02-01-preview/Microsoft.Sql.json

@@ -1385,13 +1385,13 @@
           "items": {
             "oneOf": [
               {
-                "$ref": "#/definitions/servers_databases_extensions_childResource"
+                "$ref": "#/definitions/servers_databases_backupShortTermRetentionPolicies_childResource"
               },
               {
-                "$ref": "#/definitions/servers_databases_ledgerDigestUploads_childResource"
+                "$ref": "#/definitions/servers_databases_extensions_childResource"
               },
               {
-                "$ref": "#/definitions/servers_databases_backupShortTermRetentionPolicies_childResource"
+                "$ref": "#/definitions/servers_databases_ledgerDigestUploads_childResource"
               },
               {
                 "$ref": "#/definitions/servers_databases_extendedAuditingSettings_childResource"
@@ -3179,6 +3179,17 @@
     "BackupShortTermRetentionPolicyProperties": {
       "type": "object",
       "properties": {
+        "diffBackupIntervalInHours": {
+          "oneOf": [
+            {
+              "type": "integer"
+            },
+            {
+              "$ref": "https://schema.management.azure.com/schemas/common/definitions.json#/definitions/expression"
+            }
+          ],
+          "description": "The differential backup interval in hours. This is how many interval hours between each differential backup will be supported. This is only applicable to live databases but not dropped databases."
+        },
         "retentionDays": {
           "oneOf": [
             {