qt: fix stupid Qt 6.6 bug...

This commit made that property FINAL without proper justification... :

https://github.com/qt/qtdeclarative/commit/351979e05ad2a861fc3e6f8d1de6197a751316a8

Then they realized their mistake and reverted it:

https://github.com/qt/qtdeclarative/commit/bd131dcbf579f822ac8cee98324dc1355f7cd314

The revert is directed to be picked to the 6.6 branch. But it apparently got never realized... Qt 6.6.3, which is the last Qt 6.6 version, still does not have the revert.

Close #29032 (closed).

Request review @chub.