TaskTree: Rename optional into finishAllAndDone

Rationale:
1. More descriptive.
2. More consistent with a planned new finishAllAndError policy.
3. Limits the possibilities of making a conflict with std::optional.

Change-Id: I5155630188e4b699e6c18b13a101e0e2d4fe98f2
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/remotelinux/linuxdevicetester.cpp

@@ -131,7 +131,7 @@ TaskItem GenericLinuxDeviceTesterPrivate::unameTask() const
             emit q->errorMessage(Tr::tr("uname failed.") + '\n');
     };
     return Group {
-        optional,
+        finishAllAndDone,
         ProcessTask(setup, done, error)
     };
 }
@@ -159,7 +159,7 @@ TaskItem GenericLinuxDeviceTesterPrivate::gathererTask() const
     };
 
     return Group {
-        optional,
+        finishAllAndDone,
         DeviceUsedPortsGathererTask(setup, done, error)
     };
 }