Refactor `pallet-grandpa` to use `v2` benchmarks (#6073)

# Description

This PR moves the `pallet-grandpa` to the `v2` of `frame_benchmarking`.
I submitted PR #6025 as an external contributor from my own fork, so I
made this one from within this repo to see how the process would change.

## Integration

N/A

## Review Notes

Same as #6025, straightforward.

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Co-authored-by: Shawn Tabrizi <[email protected]>