Cannot download SITL firmware to start simulation

#### Issue details
While trying to download a SITL image for the plane in the simulation tab, the image download is failing.

Debian 11/amd64
Mono 6.12

#### Version
1.3.77 official

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[x] Plane
[ ] Rover
[ ] Sub

#### Airframe type
simu

#### Hardware type
simu

#### Logs
```
INFO MissionPlanner.ArduPilot.APFirmware - device: {
"name": null,
"description": null,
"board": "",
"hardwareid": ""
}
INFO MissionPlanner.Utilities.Download - https://firmware.ardupilot.org/Plane/stable/SITL_x86_64_linux_gnu/arduplane
INFO MissionPlanner.Utilities.Download - getFilefromNet(): System.Net.WebException: Error: TrustFailure (Authentication failed, see inner exception.) ---> System.Security.Authentication.AuthenticationException: Authentication failed, see inner exception. ---> Mono.Btls.MonoBtlsException: Ssl error:1000007d:SSL routines:OPENSSL_internal:CERTIFICATE_VERIFY_FAILED
at /build/mono-6.12.0.122/external/boringssl/ssl/handshake_client.c:1132
at Mono.Btls.MonoBtlsContext.ProcessHandshake () [0x00048] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at Mono.Net.Security.MobileAuthenticatedStream.ProcessHandshake (Mono.Net.Security.AsyncOperationStatus status, System.Boolean renegotiate) [0x000da] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at (wrapper remoting-invoke-with-check) Mono.Net.Security.MobileAuthenticatedStream.ProcessHandshake(Mono.Net.Security.AsyncOperationStatus,bool)
at Mono.Net.Security.AsyncHandshakeRequest.Run (Mono.Net.Security.AsyncOperationStatus status) [0x00006] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at Mono.Net.Security.AsyncProtocolRequest.ProcessOperation (System.Threading.CancellationToken cancellationToken) [0x000fc] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
--- End of inner exception stack trace ---
at Mono.Net.Security.MobileAuthenticatedStream.ProcessAuthentication (System.Boolean runSynchronously, Mono.Net.Security.MonoSslAuthenticationOptions options, System.Threading.CancellationToken cancellationToken) [0x00262] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at Mono.Net.Security.MonoTlsStream.CreateStream (System.Net.WebConnectionTunnel tunnel, System.Threading.CancellationToken cancellationToken) [0x0016a] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at System.Net.WebConnection.CreateStream (System.Net.WebOperation operation, System.Boolean reused, System.Threading.CancellationToken cancellationToken) [0x001ba] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
--- End of inner exception stack trace ---
at System.Net.WebConnection.CreateStream (System.Net.WebOperation operation, System.Boolean reused, System.Threading.CancellationToken cancellationToken) [0x0021a] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at System.Net.WebConnection.InitConnection (System.Net.WebOperation operation, System.Threading.CancellationToken cancellationToken) [0x00141] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at System.Net.WebOperation.Run () [0x0009a] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at System.Net.WebCompletionSource`1[T].WaitForCompletion () [0x00094] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at System.Net.HttpWebRequest.RunWithTimeoutWorker[T] (System.Threading.Tasks.Task`1[TResult] workerTask, System.Int32 timeout, System.Action abort, System.Func`1[TResult] aborted, System.Threading.CancellationTokenSource cts) [0x000f8] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at System.Net.HttpWebRequest.GetResponse () [0x00016] in <9c6e2cb7ddd8473fa420642ddcf7ce48>:0
at MissionPlanner.Utilities.Download.getFilefromNet (System.String url, System.String saveto, System.Action`2[T1,T2] status) [0x0006f] in <78bc2f1b850e4a7a8aab0cc60aa4bdfa>:0
CustomMessageBox.Show
CustomMessageBox thread calling Base Thread
CustomMessageBox thread running Base Thread
```

Start at MissionPlanner.Utilities.Download.getFilefromNet, the entry point named in the log, and reproduce the SITL Plane download on Debian 11 with Mono 6.12. Trace the certificate verification failure for the firmware.ardupilot.org URL; done means the SITL image downloads successfully in the simulation tab without the TrustFailure exception.

Written by the indexing model from the issue text.