[Linux] Unabled to login to retroachievement on Ubuntu 26.04 LTS (mono-6.14.1)

Summary

Attempting to login to retroachievement on Ubuntu 26.04 will always result in a "Login Failed" dialog box and the resulting error, shown below.

I believe this is due to a mono package regression in Ubuntu 26.04 which now defaults to mono-6.14.1 mono-complete now resolves to mono-devel and ca-certificates-mono along with a number of other binaries are no longer shipped.

Repro

1. Be on Ubuntu 26.04 or later, and Install mono-compelete
2. Attempt to login to retroachievement

Output

Using SDL2 for host input (keyboard + gamepads)
Loaded RCheevos v11.6
Connected SDL gamepad, device index 0, instance ID 1, name 6B controller
System.Net.Http.HttpRequestException: The SSL connection could not be established, 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 ./external/boringssl/ssl/handshake_client.c:1132
  at Mono.Btls.MonoBtlsContext.ProcessHandshake () [0x00064] in <bc966b367c7c46aa936bfc6656db2316>:0 
  at Mono.Net.Security.MobileAuthenticatedStream.ProcessHandshake (Mono.Net.Security.AsyncOperationStatus status, System.Boolean renegotiate) [0x00106] in <bc966b367c7c46aa936bfc6656db2316>: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 <bc966b367c7c46aa936bfc6656db2316>:0 
  at Mono.Net.Security.AsyncProtocolRequest.ProcessOperation (System.Threading.CancellationToken cancellationToken) [0x0012a] in <bc966b367c7c46aa936bfc6656db2316>: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) [0x00346] in <bc966b367c7c46aa936bfc6656db2316>:0 
  at System.Net.Http.ConnectHelper.EstablishSslConnectionAsyncCore (System.IO.Stream stream, System.Net.Security.SslClientAuthenticationOptions sslOptions, System.Threading.CancellationToken cancellationToken) [0x000d2] in <194d75dbcf11427f83c01d6371e21cd1>:0 
   --- End of inner exception stack trace ---
  at System.Net.Http.ConnectHelper.EstablishSslConnectionAsyncCore (System.IO.Stream stream, System.Net.Security.SslClientAuthenticationOptions sslOptions, System.Threading.CancellationToken cancellationToken) [0x00117] in <194d75dbcf11427f83c01d6371e21cd1>:0 
  at System.Net.Http.HttpConnectionPool.CreateConnectionAsync (System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken) [0x0038d] in <194d75dbcf11427f83c01d6371e21cd1>:0 
  at System.Net.Http.HttpConnectionPool.WaitForCreatedConnectionAsync (System.Threading.Tasks.ValueTask`1[TResult] creationTask) [0x000e8] in <194d75dbcf11427f83c01d6371e21cd1>:0 
  at System.Net.Http.HttpConnectionPool.SendWithRetryAsync (System.Net.Http.HttpRequestMessage request, System.Boolean doRequestAuth, System.Threading.CancellationToken cancellationToken) [0x00085] in <194d75dbcf11427f83c01d6371e21cd1>:0 
  at System.Net.Http.RedirectHandler.SendAsync (System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken) [0x000b8] in <194d75dbcf11427f83c01d6371e21cd1>:0 
  at System.Net.Http.HttpClient.FinishSendAsyncBuffered (System.Threading.Tasks.Task`1[TResult] sendTask, System.Net.Http.HttpRequestMessage request, System.Threading.CancellationTokenSource cts, System.Boolean disposeCts) [0x0018f] in <194d75dbcf11427f83c01d6371e21cd1>:0 
  at BizHawk.Client.EmuHawk.RCheevos.HttpPost (System.String url, System.String post, System.String type) [0x000ab] in <923595f18a5244b8a77a3faa19fe4777>:0 
Disconnecting SDL gamepad, device index 0, instance ID 1, name 6B controller
abort_threads: Failed aborting id: 0x7512cbbfe6c0, mono_thread_manage will ignore it

Host env.

• Ubuntu 26.04 LTS (resolute)
• BizHawk 2.11.1 https://github.com/TASEmulators/BizHawk/commit/bdddf4a58aa1a022afb11dc73294a81a5aa7bbd5
• Mono: 6.14.1+ds2-2 (mono-devel, from Ubuntu package manager)

Start by reproducing the RetroAchievements login on Ubuntu 26.04 with Mono 6.14.1 and inspect the RCheevos.HttpPost entry point shown in the stack trace. Compare the certificate-verification failure with a working environment; done means login succeeds on the reported Ubuntu and Mono setup without weakening certificate validation.