[polkadot-runtime-parachains] migrate disputes and disputes/slashing to benchmarking to bench v2 syntax (#6577)

[polkadot-runtime-parachains] migrate disputes and disputes/slashing to
benchmarking to bench v2 syntax

Part of:
* #6202

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Co-authored-by: Giuseppe Re <giuseppe.re@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>