Update contracts to support Solidity 0.8.x (#2442)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/utils/Counters.sol

@@ -1,8 +1,6 @@
 // SPDX-License-Identifier: MIT
 
-pragma solidity >=0.6.0 <0.8.0;
-
-import "../math/SafeMath.sol";
+pragma solidity ^0.8.0;
 
 /**
  * @title Counters
@@ -11,13 +9,8 @@ import "../math/SafeMath.sol";
  * of elements in a mapping, issuing ERC721 ids, or counting request ids.
  *
  * Include with `using Counters for Counters.Counter;`
- * Since it is not possible to overflow a 256 bit integer with increments of one, `increment` can skip the {SafeMath}
- * overflow check, thereby saving gas. This does assume however correct usage, in that the underlying `_value` is never
- * directly accessed.
  */
 library Counters {
-    using SafeMath for uint256;
-
     struct Counter {
         // This variable should never be directly accessed by users of the library: interactions must be restricted to
         // the library's function. As of Solidity v0.5.2, this cannot be enforced, though there is a proposal to add
@@ -30,11 +23,10 @@ library Counters {
     }
 
     function increment(Counter storage counter) internal {
-        // The {SafeMath} overflow check can be skipped here, see the comment at the top
         counter._value += 1;
     }
 
     function decrement(Counter storage counter) internal {
-        counter._value = counter._value.sub(1);
+        counter._value = counter._value - 1;
     }
 }