Change deprecation information for QtFuture::makeReadyFuture

https://doc-snapshots.qt.io/qt6-6.6/qtfuture-obsolete.html list deprecated methods in the QtFuture namespace. Anyhow, it claims that the methods are deprecated in Qt 6.10, which is confusing. If they are deprecated only in Qt 6.10, we shouldn't list them as deprecated in Qt 6.6 documentation.

My assumption is that the 'deprecated in Qt 6.10' is mostly relevant for the compiler warnings, where people need ample time to fix their sources, and deprecating an API without some replacement API being available for some longer time is considered unfriendly. Anyhow, this is not an issue for documentation. We should therefore just mark the methods as deprecated in Qt 6.6.