Fix column droping for MSSQL that need new transaction for that (#4440)

models/migrations/v64.go

 		}
 	}
 
+	// Commit and begin new transaction for dropping columns
+	if err := sess.Commit(); err != nil {
+		return err
+	}
+	if err := sess.Begin(); err != nil {
+		return err
+	}
+
 	if err := dropTableColumns(sess, "issue", "assignee_id"); err != nil {
 		return err
 	}

models/migrations/v69.go

 		}
 	}
 
+	// Commit and begin new transaction for dropping columns
+	if err := sess.Commit(); err != nil {
+		return err
+	}
+	if err := sess.Begin(); err != nil {
+		return err
+	}
+
 	if err := dropTableColumns(sess, "team", "unit_types"); err != nil {
 		return err
 	}