feat: Migrate Polkadot SDK to stable2603

[ukint-vs]

Summary

• migrate the workspace to Polkadot SDK stable2603
• keep the SDK source graph coherent and use upstream sc-mixnet from the same SDK source
• forward proposer proof recorders and extra extensions through custom block production
• port affected runtime, node, payment, staking-rewards, and test helpers to stable2603 APIs

Review fixes

• fixes the runtime build failure caused by incoherent polkadot-sdk feature/source resolution
• fixes dropped storage_proof_recorder and extra_extensions in gear-authorship proposer flow

Verification

• cargo metadata --format-version 1 --locked
• cargo tree -p gear-cli --all-targets --locked -i sc-mixnet
• cargo check -p vara-runtime --features std --locked
• cargo check --locked --all-targets -p gear-service -p gear-authorship -p pallet-gear-payment -p pallet-gear-staking-rewards -p gear-sandbox -p gear-sandbox-interface
• cargo nextest run --locked --no-fail-fast -p gear-authorship -p pallet-gear-payment -p pallet-gear-staking-rewards
• make workspace-hack
• make fmt-check