Fix staked amount too high after unstake issue

[Dinonard]

Issue Description

Fixes the issue where the unstake call wouldn't correctly update future era staked amount.
E.g. for a situation where we have:

• staked: { era: 10, amount: 20 }, future_staked: { era: 11, amount: 21 }
• staker calls unstake(era: 10, amount: 5)
• state is incorrectly updated to: staked: {era: 10, amount: 15}, future_staked: {era: 11, amount: 21}
• the future era amount isn't correctly reduced, resulting in ContractStake remaining too high for a contract

Consequence

dApps might have ended up in higher tier than they were supposed to.
No actions will be taken around this - whatever dApps earned, it's theirs.

Fix

When unstake operation is performed on SingularStakingInfo struct, it will now correctly account for the scenario where potential future era amount also needs to be reduced.

[github-actions]

Package	Line Rate	Branch Rate	Health
precompiles/sr25519/src	64%	0%	➖
precompiles/xvm/src	75%	0%	➖
pallets/dapp-staking-v3/src/test	0%	0%	❌
precompiles/substrate-ecdsa/src	74%	0%	➖
pallets/dapp-staking-v3/src/benchmarking	98%	0%	✔
precompiles/dapp-staking-v3/src/test	0%	0%	❌
precompiles/dapp-staking-v3/src	90%	0%	✔
pallets/collator-selection/src	92%	0%	✔
chain-extensions/types/xvm/src	0%	0%	❌
precompiles/dispatch-lockdrop/src	86%	0%	✔
pallets/inflation/src	83%	0%	✔
pallets/xc-asset-config/src	64%	0%	➖
precompiles/xcm/src	73%	0%	➖
primitives/src	62%	0%	➖
pallets/dynamic-evm-base-fee/src	92%	0%	✔
chain-extensions/xvm/src	0%	0%	❌
precompiles/assets-erc20/src	81%	0%	✔
pallets/astar-xcm-benchmarks/src/generic	100%	0%	✔
chain-extensions/pallet-assets/src	56%	0%	➖
pallets/unified-accounts/src	86%	0%	✔
pallets/oracle-benchmarks/src	0%	0%	❌
pallets/static-price-provider/src	52%	0%	➖
pallets/dapp-staking-migration/src	0%	0%	❌
chain-extensions/unified-accounts/src	0%	0%	❌
pallets/astar-xcm-benchmarks/src	88%	0%	✔
pallets/price-aggregator/src	72%	0%	➖
chain-extensions/types/assets/src	0%	0%	❌
pallets/xvm/src	54%	0%	➖
pallets/dapp-staking-v3/src	90%	0%	✔
chain-extensions/types/unified-accounts/src	0%	0%	❌
precompiles/unified-accounts/src	100%	0%	✔
pallets/astar-xcm-benchmarks/src/fungible	100%	0%	✔
pallets/ethereum-checked/src	79%	0%	➖
pallets/dapp-staking-v3/rpc/runtime-api/src	0%	0%	❌
primitives/src/xcm	64%	0%	➖
Summary	77% (3647 / 4717)	0% (0 / 0)	➖

Minimum allowed line rate is 50%