QCborStreamWriter: use (new) QUtf8::convertFromLatin1() to speed up a…

…ppend(QL1SV)

Use the new function to convert into a QVLA instead of first
converting to a QString and then to a QByteArray. One conversion and
one buffer instead of two each.

Pick-to: 6.6
Change-Id: Ieaa24c8ef325797b5a89e8da0ca4310667c00fa7
Reviewed-by: Thiago Macieira <[email protected]>

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