Migrating polkadot-runtime-common slots benchmarking to v2 (#6614)

#Description
Migrated polkadot-runtime-parachains slots benchmarking to the new
benchmarking syntax v2.
This is part of #6202

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Co-authored-by: Giuseppe Re <giuseppe.re@parity.io>
Co-authored-by: seemantaggarwal <32275622+seemantaggarwal@users.noreply.github.com>
Co-authored-by: Bastian Köcher <git@kchr.de>