[RDBMS] az postgres flexible-server upgrade: Allow major version upgrade to PostgreSQL Version 18 (#32565)

Author: nasc17

src/azure-cli/azure/cli/command_modules/rdbms/_helptext_pg.py

@@ -40,7 +40,7 @@
         az postgres flexible-server create --location northeurope --resource-group testGroup \\
           --name testserver --admin-user username --admin-password password \\
           --sku-name Standard_D2s_v3 --tier GeneralPurpose --public-access 153.24.26.117 --storage-size 128 \\
-          --tags "key=value" --version 17 --zonal-resiliency Enabled --zone 1 \\
+          --tags "key=value" --version 18 --zonal-resiliency Enabled --zone 1 \\
           --standby-zone 3
   - name: >
       Create server with high availability feature enabled that allows primary and standby in the same zone when multi-zone capacity is unavailable.
@@ -910,8 +910,8 @@
 type: command
 short-summary: Upgrade the major version of a flexible server.
 examples:
-  - name: Upgrade server 'testsvr' to PostgreSQL major version 17.
-    text: az postgres flexible-server upgrade -g testgroup -n testsvr -v 17
+  - name: Upgrade server 'testsvr' to PostgreSQL major version 18.
+    text: az postgres flexible-server upgrade -g testgroup -n testsvr -v 18
 """
 
 helps['postgres flexible-server identity'] = """

src/azure-cli/azure/cli/command_modules/rdbms/_params.py

@@ -472,7 +472,7 @@ def _flexible_server_params(command_group):
         )
 
         pg_version_upgrade_arg_type = CLIArgumentType(
-            arg_type=get_enum_type(['13', '14', '15', '16', '17']),
+            arg_type=get_enum_type(['13', '14', '15', '16', '17', '18']),
             options_list=['--version', '-v'],
             help='Server major version.'
         )