MilestoneVotes migration

[f-gate]

The problem is that this bit of code will be used in the rutnime api to collect total votes.
This means that we will iterate over the keys of a Project key, thereby doing up to T::MaxMilestonesPerProject iterations which is obviously dreadful.

    #[pallet::storage]
    #[pallet::getter(fn milestone_votes)]
    pub(super) type MilestoneVotes<T: Config> = StorageDoubleMap<
        _,
        Identity,
        ProjectKey,
        Identity,
        MilestoneKey,
        Vote<BalanceOf<T>>,
        OptionQuery,
    >;

Suggested change:

    #[pallet::storage]
    #[pallet::getter(fn milestone_votes)]
    pub(super) type MilestoneVotes<T: Config> = StorageMap<
        _,
        Identity,
        ProjectKey,
        BoundedBTreeMap<MilestoneKey, Vote<BalanceOf<T>>, T::MaxMilestonesPerProject>,
        ValueQuery,
    >;