osguard: set size to 24GB to successfully repart (#14453)

The osguard repartitioning configuration in repart.d ends up repartitioning the disk to be 23.8 GB in size. The starting image is only 5.8 GB, so when the image is uploaded to Azure, OS disks created will start at 6 GB in size, unless directly specified to be greater.

```
root [ /home/azureuser ]# systemctl status systemd-repart
Warning: The unit file, source configuration file or drop-ins of systemd-repart.service changed on disk. Run 'systemctl daemon-reload' to reload units.
× systemd-repart.service - Repartition Root Disk
     Loaded: loaded (/usr/lib/systemd/system/systemd-repart.service; static)
    Drop-In: /usr/lib/systemd/system/service.d
             └─10-timeout-abort.conf
     Active: failed (Result: exit-code) since Wed 2025-08-06 23:14:04 UTC; 3h 53min ago
       Docs: man:systemd-repart.service(8)
   Main PID: 851 (code=exited, status=1/FAILURE)
        CPU: 17ms

Aug 06 23:14:04 azure-linux-os-guard systemd[1]: Starting systemd-repart.service - Repartition Root Disk...
Aug 06 23:14:04 azure-linux-os-guard systemd-repart[851]: Can't fit requested partitions into available free space (0B), refusing.
Aug 06 23:14:04 azure-linux-os-guard systemd-repart[851]: Automatically determined minimal disk image size as 23.8G, current image size is 6.0G.
Aug 06 23:14:04 azure-linux-os-guard systemd[1]: systemd-repart.service: Main process exited, code=exited, status=1/FAILURE
Aug 06 23:14:04 azure-linux-os-guard systemd[1]: systemd-repart.service: Failed with result 'exit-code'.
Aug 06 23:14:04 azure-linux-os-guard systemd[1]: Failed to start systemd-repart.service - Repartition Root Disk.
```

To avoid repartitioning service errors, set the default starting disk size to an adequate value so repartitioning always is successful.

Signed-off-by: Chris Co <[email protected]>

Author: christopherco

toolkit/imageconfigs/osguard-amd64.yaml

@@ -3,6 +3,7 @@ storage:
 
   disks:
     - partitionTableType: gpt
+      maxSize: 24G
       partitions:
         - id: esp
           type: esp