AURA: Switch to `CurrentSlot` instead of `LastTimestamp` (#8023)

* Convert AURA to new pallet macro

* AURA: Switch to `CurrentSlot` instead of `LastTimestamp`

This switches AURA to use `CurrentSlot` instead of `LastTimestamp`.

* Add missing file

* Update frame/aura/src/migrations.rs

Co-authored-by: André Silva <[email protected]>

* Remove the runtime side provide inherent code

* Use correct weight

* Add TODO

* Remove the Inherent from AURA

* 🤦



* Remove unused stuff

Co-authored-by: André Silva <[email protected]>