Export-DbaUser - Use server version instead of db compat level for scripting

When no -DestinationVersion is specified, use the actual SQL Server instance
version (VersionMajor) rather than the database compatibility level as the
SMO scripting target. This fixes an error where databases on newer SQL Server
instances (e.g., 2022) with a lower compatibility level (e.g., 120/SQL2014)
could not export users that use features not available at the compat level,
such as External users introduced in SQL Server 2016.

Fixes #10343

(do Export-DbaUser)

Co-authored-by: Andreas Jordan <andreasjordan@users.noreply.github.com>

Author: github-actions[bot]

public/Export-DbaUser.ps1

@@ -216,6 +216,21 @@ function Export-DbaUser {
             'Version160' = 'SQLServer2022'
         }
 
+        # Maps SQL Server major version number to SMO scripting version string.
+        # Used to resolve the actual server version when no DestinationVersion is specified,
+        # since the database compatibility level may be lower than the server version.
+        $serverVersionMap = @{
+            8  = 'Version80'
+            9  = 'Version90'
+            10 = 'Version100'
+            11 = 'Version110'
+            12 = 'Version120'
+            13 = 'Version130'
+            14 = 'Version140'
+            15 = 'Version150'
+            16 = 'Version160'
+        }
+
         $eol = [System.Environment]::NewLine
 
     }
@@ -232,8 +247,17 @@ function Export-DbaUser {
         foreach ($db in $InputObject) {
 
             if ([string]::IsNullOrEmpty($destinationVersion)) {
-                #Get compatibility level for scripting the objects
-                $scriptVersion = $db.CompatibilityLevel
+                # Use the actual server version rather than the database compatibility level.
+                # The compatibility level may be lower than the server version (e.g., a SQL 2022
+                # instance hosting a database at compat level 120/SQL2014). Scripting against
+                # the lower compat level causes errors for features like External users that are
+                # supported by the server but not recognised by older scripting targets.
+                $serverMajorVersion = $db.Parent.VersionMajor
+                if ($serverVersionMap.ContainsKey($serverMajorVersion)) {
+                    $scriptVersion = $serverVersionMap[$serverMajorVersion]
+                } else {
+                    $scriptVersion = $db.CompatibilityLevel
+                }
             } else {
                 $scriptVersion = $versions[$destinationVersion]
             }