All Qt 6 containers have "fast" prepend these days. Except
QVLA. Instead of enabling "fast" prepend for QVLA, slowing down
idiomatic QVLA use, simply deprecate prepend().
There appear to be no users of this function in qtbase outside tests.
[ChangeLog][QtCore][Deprecation Notices][QVarLengthArray] Deprecated
prepend() because QVarLengthArray is the only Qt container without a
"fast" prepend. If you require that functionality, even though it's a
linear operation, then use insert(cbegin(), ~~~) instead.
Change-Id: I39ff1dd7d4de7fc08d5380a5a7450dd8c8996fe2
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
85e92f2e5f