Use standaloneMonthName in the Month View's header

Use `standaloneMonthName()` to generate the month name in the Month
View's header, as is done by KOrganizer's date navigator, because
`QString::toString()` produces inappropriate month names for Slavic
languages in this context.

BUG: 453292
2 jobs for !68 with month-name in 55 seconds (queued for 2 seconds)
latest merge request