Update dg-readiness-tool.md

try+catch is needed to handle "Variable is currently undefined." error on some older computers

Confirm-SecureBootUEFI : Variable is currently undefined: 0xC0000100
At line:1 char:1
+ Confirm-SecureBootUEFI
+ ~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ResourceUnavailable: (Microsoft.Secur...BootUefiCommand:ConfirmSecureBootUefiCommand) [C
   onfirm-SecureBootUEFI], StatusException
    + FullyQualifiedErrorId : GetFWVarFailed,Microsoft.SecureBoot.Commands.ConfirmSecureBootUefiCommand

https://docs.microsoft.com/en-us/powershell/module/secureboot/get-securebootuefi?view=windowsserver2022-ps

windows/security/identity-protection/credential-guard/dg-readiness-tool.md

@@ -796,7 +796,13 @@ function CheckOSArchitecture
 
 function CheckSecureBootState
 {
-    $_secureBoot = Confirm-SecureBootUEFI
+    try { 
+        $_secureBoot = Confirm-SecureBootUEFI
+    }
+    catch
+    {
+        $_secureBoot = $false
+    }
     Log $_secureBoot
     if($_secureBoot)
     {