Add storage bounds for pallet staking

[re-gius]

This is part of #6289 and necessary for the Asset Hub migration.

Building on the observations and suggestions from #255 .

Changes

• Add MaxInvulnerables to bound Invulnerables Vec.
  • Set to constant 4 in the pallet.
• Add MaxActiveValidators to bound DisabledValidators Vec and EraRewardPoints.individual BTreeMap.
  • Set to already existing MaxActiveValidators in westend runtime.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/11976930015
Failed job name: cargo-clippy