Refactor staking pallet benchmarks to `v2` (#6025)

# Description
This PR migrates the staking pallet's benchmarks to the `v2` of pallet
benchmarking tooling provided by
[`frame_benchmarking`](https://github.com/paritytech/polkadot-sdk/tree/master/substrate/frame/benchmarking).

## Integration

N/A

## Review Notes

The PR is straightforward, as were #1676 , #1838 and #1868.

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Co-authored-by: Dónal Murray <donal.murray@parity.io>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>