Support PG18 virtual generated columns

Closes #3568

Author: roji

src/EFCore.PG/Migrations/NpgsqlMigrationsSqlGenerator.cs

@@ -1872,10 +1872,11 @@ protected override void ComputedColumnDefinition(
             throw new NotSupportedException("Computed/generated columns aren't supported in PostgreSQL prior to version 12");
         }
 
-        if (operation.IsStored != true)
+        if (operation.IsStored is not true && _postgresVersion < new Version(18, 0))
         {
             throw new NotSupportedException(
-                "Generated columns currently must be stored, specify 'stored: true' in "
+                "Virtual (non-stored) generated columns are only supported on PostgreSQL 18 and up. " +
+                "On older versions, specify 'stored: true' in "
                 + $"'{nameof(RelationalPropertyBuilderExtensions.HasComputedColumnSql)}' in your context's OnModelCreating.");
         }
 
@@ -1894,12 +1895,17 @@ protected override void ComputedColumnDefinition(
         builder
             .Append(" GENERATED ALWAYS AS (")
             .Append(operation.ComputedColumnSql!)
-            .Append(") STORED");
+            .Append(")");
 
-        if (!operation.IsNullable)
+        if (operation.IsStored is true)
         {
-            builder.Append(" NOT NULL");
+            builder.Append(" STORED");
         }
+
+        if (!operation.IsNullable)
+            {
+                builder.Append(" NOT NULL");
+            }
     }
 
 #pragma warning disable 618