Migrate pallet-democracy benchmarks to benchmark v2 syntax (#6509)

# Description

Migrates pallet-democracy benchmarks to benchmark v2 syntax
This is Part of https://github.com/paritytech/polkadot-sdk/issues/6202

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: command-bot <>
Co-authored-by: Dmitry Markin <dmitry@markin.tech>
Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

prdoc/pr_6509.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: Migrate pallet-democracy benchmark to v2
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      "Part of issue #6202."
+
+crates:
+- name: pallet-democracy
+  bump: patch