enable approval-voting-parallel on polkadot (#7504)

approval-voting-parallel has been running on all network types except
polkadot since release `1.17.0`, so it has around 2 months of baking and
running without any reported issues, so let's enable it on polkadot as
well.

After running in polkadot for awhile the flag will be entirely removed.

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Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>
(cherry picked from commit e5e0a6d3)