Chore: Cleanup `CrowdloanClaim` and `CrowdloanReward` storage (requires Polkadot SDK v1.9.0+)

[wischli]

Description

After upgrading to Polkadot SDK v1.9.0+, use Multiblock migrations to cleanup the following pallets which have already been removed:

• CrowdloanClaim
• CrowdloanReward

Research/based on

• Leftover from Remove crowdloan pallets #1834

How will this affect the code base

• Onchain storage will be reduced

What are foreseen obstacles or hurdles to overcome?

• Keeping dead storage does not impose security risk
• Requires multi block migration