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.

---------

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

prdoc/pr_6025.prdoc

+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Refactor staking pallet benchmarks to `v2`
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Update benchmarks in staking pallet to the second version of the `frame_benchmarking` runtime benchmarking framework.
+
+crates:
+  - name: pallet-staking
+    bump: patch
\ No newline at end of file