migrate pallet-elections-phragmen to attribute macros (#8044)

* All done

* Fix benchmarks

* Apply suggestions from code review

Co-authored-by: Guillaume Thiolliere <[email protected]>

* Fix metadata.

* Fix build

* Add migrations

* Fix

* Update frame/elections-phragmen/src/migrations/v4.rs

* Better migeation test

* More test

* Fix warn

* Update frame/elections-phragmen/src/lib.rs

Co-authored-by: Guillaume Thiolliere <[email protected]>

* Fix test

* early exit

* Fix

* Fix build

Co-authored-by: Guillaume Thiolliere <[email protected]>