fix: mark some non-deployable contracts as abstract (C4 QA)

Author: pcarranzav

contracts/governance/Governed.sol

@@ -6,7 +6,7 @@ pragma solidity ^0.7.6;
  * @title Graph Governance contract
  * @dev All contracts that will be owned by a Governor entity should extend this contract.
  */
-contract Governed {
+abstract contract Governed {
     // -- State --
 
     /// Address of the governor

contracts/governance/Managed.sol

@@ -22,7 +22,7 @@ import "./IManaged.sol";
  * Inspired by Livepeer:
  * https://github.com/livepeer/protocol/blob/streamflow/contracts/Controller.sol
  */
-contract Managed is IManaged {
+abstract contract Managed is IManaged {
     // -- State --
 
     /// Controller that contract is registered with

contracts/governance/Pausable.sol

@@ -2,7 +2,7 @@
 
 pragma solidity ^0.7.6;
 
-contract Pausable {
+abstract contract Pausable {
     /**
      * @dev "Partial paused" pauses exit and enter functions for GRT, but not internal
      * functions, such as allocating

contracts/l2/token/GraphTokenUpgradeable.sol

@@ -24,7 +24,7 @@ import "../../governance/Governed.sol";
  * initializer functions and upgradeable OpenZeppelin contracts instead of
  * the original's constructor + non-upgradeable approach.
  */
-contract GraphTokenUpgradeable is GraphUpgradeable, Governed, ERC20BurnableUpgradeable {
+abstract contract GraphTokenUpgradeable is GraphUpgradeable, Governed, ERC20BurnableUpgradeable {
     // -- EIP712 --
     // https://github.com/ethereum/EIPs/blob/master/EIPS/eip-712.md#definition-of-domainseparator
 

contracts/upgrades/GraphProxyStorage.sol

@@ -8,7 +8,7 @@ pragma solidity ^0.7.6;
  * This contract does not actually define state variables managed by the compiler
  * but uses fixed slot locations.
  */
-contract GraphProxyStorage {
+abstract contract GraphProxyStorage {
     /**
      * @dev Storage slot with the address of the current implementation.
      * This is the keccak-256 hash of "eip1967.proxy.implementation" subtracted by 1, and is

contracts/upgrades/GraphUpgradeable.sol

@@ -8,7 +8,7 @@ import "./IGraphProxy.sol";
  * @title Graph Upgradeable
  * @dev This contract is intended to be inherited from upgradeable contracts.
  */
-contract GraphUpgradeable {
+abstract contract GraphUpgradeable {
     /**
      * @dev Storage slot with the address of the current implementation.
      * This is the keccak-256 hash of "eip1967.proxy.implementation" subtracted by 1, and is