pallet-bounties: allow bounties to never expire (#7723)

# Description

Fixes polkadot-fellows/runtimes#509

The Bounties expiration and renewal heavily depends on manual
interactions through UI. This PR refactors the duration of the bounty to
be an optional configuration constant. If set to None, bounties remain
active indefinitely, removing the need for calling`extend_bounty_expiry`
and preventing automatic curator slashing for inactivity, which often
penalises unnecessarily.

## Integration

Remove
[BountyUpdatePeriod](https://github.com/polkadot-fellows/runtimes/blob/db4bb534cb411c0d6a2fe57eb331e6ec93ace825/relay/polkadot/src/lib.rs#L774)

## Review Notes

Modifies how bounty expiry is handled
<details>
<summary> Code Diff Summary</summary>

```diff
- #[pallet::constant]
- type BountyUpdatePeriod: Get<BlockNumberFor<Self, I>>;
+ #[pallet::constant]
+ type BountyUpdatePeriod: Get<Option<BlockNumberFor<Self, I>>>;

- *update_due = (Self::treasury_block_number() + T::BountyUpdatePeriod::get()).max(*update_due);
+ *update_due = Self::treasury_block_number().saturating_add(
+     T::BountyUpdatePeriod::get().unwrap_or(BlockNumberFor::<T, I>::MAX)
+ ); 
```
</details>