Stake tracker improvements (migration and try-state checks OK in Polkadot) (#4673)

Changes in the migration code introduced in this PR:
- Migration removes duplicate nominations in all nominators, if they
exist (changes by calling `fn do_add_nominator` with dedup nominations)
- Migration removes all the non active validator nominations to avoid
adding dangling nominations (changes by calling `fn do_add_nominator` if
necessary)
- Migration iterates through all `Nominators` map first and through all
the `Validators` map to insert all the validators that are not nominated
to the target list (with self-stake only).
- Moves stake-tracker related try-state checks to the staking pallet (no
try-state checks in the stake-tracker pallet anymore).
- Runs benchmarks for MMB migration step with bench bot. 

The migration code has been validated against the Polkadot using the
externalities tests in
[polkadot/runtime/westend/src/lib.rs](https://github.com/paritytech/polkadot-sdk/pull/4436/files/46a80f14

#diff-5264d812f7aadfc486efffd3322b11c8dee5e8ddf2e77e0a9e70b573f3d2afdd).
Upon running the migrations, we ensure that:
- All validators have been added to the target list with their correct
approvals score (as per the try-state checks).
- All nominations are "cleaned" (see def. of clean above)
- Try-state checks related to stake-tracker and approvals pass.

Note: Same as https://github.com/paritytech/polkadot-sdk/pull/4436 but
with a non-messed up git history. Merging into
`gpestana/stake-tracker_integration`.

---------

Co-authored-by: Ankan <[email protected]>
Co-authored-by: command-bot <>