Liquid INV rate probably too low

https://github.com/ElementsProject/elements/blob/cfc10a5dd6c18db140dd1d1e94dfa8cad16bfe36/src/net_processing.cpp#LL136C1-L136C1

```c++
/** Maximum rate of inventory items to send per second.
* Limits the impact of low-fee transaction floods. */
static constexpr unsigned int INVENTORY_BROADCAST_PER_SECOND = 7;
```

Given liquid's higher block rate, this value should probably be higher ([also probably true on Bitcoin itself](https://github.com/bitcoin/bitcoin/pull/27630)) -- perhaps it should be coded as `(4200 / consensusParams.nPowTargetSpacing)` ?