Migration cleanup and tidy up

[thadouk]

Proposed changes

This PR cleanups the migrations and fixes some other minor issues:

• Removes migrations from summary, validators manager and staking pallets and their wire up with runtime. These migrations have been completed a long time ago on all active chains. Its good practice to remove from time to time to reduce the size of the codebase and upgrade checks performed. All migrations set the right storage item on the genesis build block, so new chains are skipping the migration.
• updates grade configuration for sepolia.

Type of change/Merge

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• Release
  • Increase versions
  • Baseline tests passed
  • Release type:
    • Major release
    • Minor release
    • Patch release

Checklist

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• You describe the purpose of the PR, e.g.:
  • What does it do?
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  • Indicates if there is something left for follow-up PRs.
• Documentation updated
• Business logic tested successfully
• Verify First, Write Last: In Substrate development, it is important that you always ensure preconditions are met and return errors at the beginning. After these checks have completed, then you may begin the function's computation.

Further comments

[nahuseyoum]

👍