fix(ctb): correct GPO abi and layout (#4582)

Fixes some small errors in the GPO's API and storage layout that did not
conform to the standards for how we are ugprading the L2 system.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

packages/contracts-bedrock/.gas-snapshot

@@ -28,13 +28,14 @@ DeployerWhitelist_Test:test_owner_succeeds() (gas: 7538)
 DeployerWhitelist_Test:test_storageSlots_succeeds() (gas: 33395)
 FeeVault_Test:test_constructor_succeeds() (gas: 10647)
 FeeVault_Test:test_minWithdrawalAmount_succeeds() (gas: 10668)
-GasPriceOracle_Test:test_baseFee_succeeds() (gas: 8291)
+GasPriceOracle_Test:test_baseFee_succeeds() (gas: 8280)
+GasPriceOracle_Test:test_decimals_succeeds() (gas: 6145)
 GasPriceOracle_Test:test_gasPrice_succeeds() (gas: 8294)
-GasPriceOracle_Test:test_l1BaseFee_succeeds() (gas: 10656)
-GasPriceOracle_Test:test_overhead_succeeds() (gas: 10546)
-GasPriceOracle_Test:test_scalar_succeeds() (gas: 10611)
-GasPriceOracle_Test:test_setGasPrice_doesNotExist_reverts() (gas: 5910)
-GasPriceOracle_Test:test_setL1BaseFee_doesNotExist_reverts() (gas: 5910)
+GasPriceOracle_Test:test_l1BaseFee_succeeds() (gas: 10634)
+GasPriceOracle_Test:test_overhead_succeeds() (gas: 10614)
+GasPriceOracle_Test:test_scalar_succeeds() (gas: 10655)
+GasPriceOracle_Test:test_setGasPrice_doesNotExist_reverts() (gas: 5888)
+GasPriceOracle_Test:test_setL1BaseFee_doesNotExist_reverts() (gas: 5888)
 GovernanceToken_Test:test_approve_succeeds() (gas: 133293)
 GovernanceToken_Test:test_burnFrom_succeeds() (gas: 122733)
 GovernanceToken_Test:test_burn_succeeds() (gas: 114610)

packages/contracts-bedrock/.storage-layout

@@ -154,6 +154,8 @@
 | spacer_3_0_32 | uint256 | 3    | 0      | 32    | contracts/L2/GasPriceOracle.sol:GasPriceOracle |
 |---------------+---------+------+--------+-------+------------------------------------------------|
 | spacer_4_0_32 | uint256 | 4    | 0      | 32    | contracts/L2/GasPriceOracle.sol:GasPriceOracle |
+|---------------+---------+------+--------+-------+------------------------------------------------|
+| spacer_5_0_32 | uint256 | 5    | 0      | 32    | contracts/L2/GasPriceOracle.sol:GasPriceOracle |
 +---------------+---------+------+--------+-------+------------------------------------------------+
 
 =======================

packages/contracts-bedrock/contracts/L2/GasPriceOracle.sol

@@ -57,10 +57,17 @@ contract GasPriceOracle is Semver {
      */
     uint256 private spacer_4_0_32;
 
+    /**
+     * @custom:legacy
+     * @custom:spacer decimals
+     * @notice Spacer for backwards compatibility.
+     */
+    uint256 private spacer_5_0_32;
+
     /**
      * @notice Number of decimals used in the scalar.
      */
-    uint256 public constant decimals = 6;
+    uint256 public constant DECIMALS = 6;
 
     /**
      * @custom:semver 0.0.1
@@ -78,7 +85,7 @@ contract GasPriceOracle is Semver {
     function getL1Fee(bytes memory _data) external view returns (uint256) {
         uint256 l1GasUsed = getL1GasUsed(_data);
         uint256 l1Fee = l1GasUsed * l1BaseFee();
-        uint256 divisor = 10**decimals;
+        uint256 divisor = 10**DECIMALS;
         uint256 unscaled = l1Fee * scalar();
         uint256 scaled = unscaled / divisor;
         return scaled;
@@ -129,6 +136,16 @@ contract GasPriceOracle is Semver {
         return L1Block(Predeploys.L1_BLOCK_ATTRIBUTES).basefee();
     }
 
+    /**
+     * @custom:legacy
+     * @notice Retrieves the number of decimals used in the scalar.
+     *
+     * @return Number of decimals used in the scalar.
+     */
+    function decimals() public pure returns (uint256) {
+        return DECIMALS;
+    }
+
     /**
      * @notice Computes the amount of L1 gas used for a transaction. Adds the overhead which
      *         represents the per-transaction gas overhead of posting the transaction and state

packages/contracts-bedrock/contracts/test/GasPriceOracle.t.sol

@@ -74,6 +74,11 @@ contract GasPriceOracle_Test is CommonTest {
         assertEq(gasOracle.overhead(), l1FeeOverhead);
     }
 
+    function test_decimals_succeeds() external {
+        assertEq(gasOracle.decimals(), 6);
+        assertEq(gasOracle.DECIMALS(), 6);
+    }
+
     // Removed in bedrock
     function test_setGasPrice_doesNotExist_reverts() external {
         (bool success, bytes memory returndata) = address(gasOracle).call(