CalendarEventsPlugin: fix wrong deprecation for signal alternateDateReady in KF5 branch

The signal alternateCalendarDateReady is only used with Plasma6, ignored with also latest Plasma 5.27.

So people need to keep using alternateDateReady when developing against KF5.

See also

• misleading info at https://api.kde.org/frameworks/kdeclarative/html/classCalendarEvents_1_1CalendarEventsPlugin.html#a0eb33c8cef5dc0c947f4297ba7f7f7f5
• https://invent.kde.org/search?search=alternateDateReady&project_id=2703&group_id=1568&search_code=true&repository_ref=Plasma%2F5.27

@fusionfuture @carlschwan