Simplify borsh implementation in `packages/constrained_types/src/lib.rs`

Follow-up issue
https://github.com/cosmos/solidity-ibc-eureka/pull/857#discussion_r2712742180
https://github.com/cosmos/solidity-ibc-eureka/pull/857#discussion_r2682303133

Once borsh is updated we could use `derive` for `ConstrainedVec` too.