contracts-bedrock: use `1 gwei` over hardcoded number

Replaces a hardcoded number in `ResourceMetering`
with the syntax `1 gwei` which is easier to verify
for correctness. The following cast command shows
that the number was correct.

```
$ cast --to-unit 1000000000 gwei
1
```

packages/contracts-bedrock/contracts/L1/ResourceMetering.sol

@@ -141,7 +141,7 @@ abstract contract ResourceMetering is Initializable {
         // division by zero for L1s that don't support 1559 or to avoid excessive gas burns during
         // periods of extremely low L1 demand. One-day average gas fee hasn't dipped below 1 gwei
         // during any 1 day period in the last 5 years, so should be fine.
-        uint256 gasCost = resourceCost / Math.max(block.basefee, 1000000000);
+        uint256 gasCost = resourceCost / Math.max(block.basefee, 1 gwei);
 
         // Give the user a refund based on the amount of gas they used to do all of the work up to
         // this point. Since we're at the end of the modifier, this should be pretty accurate. Acts