Migrate Identity trait to frame prelude or the umbrella crate

[seemantaggarwal]

As seen during pallet migrations in here : #6504

The current implementation of the Identity trait in the prelude breaks as the trait Convert<u64, u64> is not implemented for polkadot_sdk_frame::prelude::Identity, which is required by ConvertRank<polkadot_sdk_frame::prelude::Identity>: polkadot_sdk_frame::traits::tokens::GetSalary<u64, u64, u64>

sp_runtime::{traits::Identity} needs to migrated to umbrella crate

See error run here:
https://github.com/paritytech/polkadot-sdk/actions/runs/12674245840/job/35322600536

and here:

https://github.com/paritytech/polkadot-sdk/actions/runs/12667087673/job/35299831553