Migrate pallet-elections to pallet attribute macro

[shaunxw]

Part of #7882.

Converts the Elections pallet to the new pallet attribute macro introduced in #6877.

Following the upgrade guidelines here: https://crates.parity.io/frame_support/attr.pallet.html#upgrade-guidelines.

⚠️ Breaking Change ⚠️

From https://crates.parity.io/frame_support/attr.pallet.html#checking-upgrade-guidelines

storages now use PalletInfo for module_prefix instead of the one given to decl_storage: Thus any use of this pallet in construct_runtime! should be careful to update name in order not to break storage or to upgrade storage (moreover for instantiable pallet). If pallet is published, make sure to warn about this breaking change.

So users of the Elections pallet must be careful about the name they used in construct_runtime!. Hence the runtime-migration label, which might not be needed depending on the configuration of the Elections pallet.

[shawntabrizi]

This pallet is not used in production, so should be totally non-breaking

[gui1117]

bot merge

[ghost]

Waiting for commit status.