Add storage gap variable to the CommitteeUpgradeable contract (#19228)

## Description 

In the case the `CommitteeUpgradeable` contract needs to include more
storage variables, a storage gap variable is needed to reserve storage
slots so the child contract state is not overwritten.

## Test plan 

Contract upgrade unit tests

bridge/evm/contracts/utils/CommitteeUpgradeable.sol

@@ -20,6 +20,8 @@ abstract contract CommitteeUpgradeable is
     /* ========== STATE VARIABLES ========== */
 
     bool private _upgradeAuthorized;
+    // upgradeablity storage gap
+    uint256[50] private __gap;
 
     /* ========== INITIALIZER ========== */