Use u16 for txCount and u32 for message_receipt_count (#578)

The `TxPointer.txIndex` is `u16`, so `txCount` can't be more than `u16`.

<img width="444" alt="image"
src="https://github.com/FuelLabs/fuel-specs/assets/18346821/abfaedd5-f475-4b64-a43e-d3c4e886b53a">

The maximum number of receipts is limited to `255` per transaction.
Allowing only `u16` transactions per block means we can have a maximum
of `255 * u16::MAX` that is less than `u32`.

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### After merging, notify other teams

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src/protocol/block-header.md

@@ -9,8 +9,8 @@ The application header is a network-agnostic block header. Different [networks](
 | `da_height`                      | `uint64`   | Height of the data availability layer up to which (inclusive) input messages are processed.                                                                                                                                                         |
 | `consensusParametersVersion`     | `uint32`   | The version of the consensus parameters used to execute this block.                                                                                                                                                                                 |
 | `stateTransitionBytecodeVersion` | `uint32`   | The version of the state transition bytecode used to execute this block.                                                                                                                                                                            |
-| `txCount`                        | `uint64`   | Number of [transaction](../tx-format/transaction.md)s in this block.                                                                                                                                                                                |
-| `message_receipt_count`          | `uint64`   | Number of [output message](../abi/receipts.md#messageout-receipt)s in this block.                                                                                                                                                                   |
+| `txCount`                        | `uint16`   | Number of [transaction](../tx-format/transaction.md)s in this block.                                                                                                                                                                                |
+| `message_receipt_count`          | `uint32`   | Number of [output message](../abi/receipts.md#messageout-receipt)s in this block.                                                                                                                                                                   |
 | `txRoot`                         | `byte[32]` | [Merkle root](./cryptographic-primitives.md#binary-merkle-tree) of [transaction](../tx-format/transaction.md)s in this block.                                                                                                                       |
 | `message_outbox_root`            | `byte[32]` | [Merkle root](./cryptographic-primitives.md#binary-merkle-tree) of [output message](../abi/receipts.md#messageout-receipt)s [`messageId`](../identifiers/utxo-id.md#message-id) in this block.                                                      |
 | `event_inbox_root`               | `byte[32]` | [Merkle root](./cryptographic-primitives.md#binary-merkle-tree) of all [events](./relayer.md) imported from L1 in this block. The order of the events added to the Merkle tree is the L1 block order, and the index of each event within each block |