Add Obsolete message to Send-MailMessage (#9178)

Add Obsolete message to Send-MailMessage

## PR Context

See https://github.com/PowerShell/PowerShell-RFC/pull/160

Co-authored-by: Steve Lee <slee@microsoft.com>
Co-authored-by: Ilya <darpa@yandex.ru>
# Conflicts:
#	test/powershell/Modules/Microsoft.PowerShell.Utility/Send-MailMessage.Tests.ps1

src/Microsoft.PowerShell.Commands.Utility/commands/utility/Send-MailMessage.cs

@@ -14,6 +14,7 @@ namespace Microsoft.PowerShell.Commands
     /// <summary>
     /// Implementation for the Send-MailMessage command.
     /// </summary>
+    [Obsolete("This cmdlet does not guarantee secure connections to SMTP servers. While there is no immediate replacement available in PowerShell, we recommend you do not use Send-MailMessage at this time. See https://aka.ms/SendMailMessage for more information.")]
     [Cmdlet(VerbsCommunications.Send, "MailMessage", HelpUri = "https://go.microsoft.com/fwlink/?LinkID=135256")]
     public sealed class SendMailMessage : PSCmdlet
     {

test/powershell/Modules/Microsoft.PowerShell.Utility/Send-MailMessage.Tests.ps1

@@ -115,7 +115,16 @@ Describe "Basic Send-MailMessage tests" -Tags CI {
         }
 
         $user = [Environment]::UserName
-        $inPassword = Select-String "^${user}:" /etc/passwd -ErrorAction SilentlyContinue
+        $powershell = [PowerShell]::Create()
+
+        $null = $powershell.AddCommand("Send-MailMessage").AddParameters($InputObject).AddParameter("ErrorAction","SilentlyContinue")
+
+        $powershell.Invoke()
+
+        $warnings = $powershell.Streams.Warning
+
+        $warnings.count | Should -BeGreaterThan 0
+        $warnings[0].ToString() | Should  -BeLike  "The command 'Send-MailMessage' is obsolete. *"
         if (-not $inPassword)
         {
             $PesterArgs["Pending"] = $true