Add introduction tag for v5.1 contracts (#5228)

Co-authored-by: Ernesto García <ernestognw@gmail.com>

contracts/utils/structs/CircularBuffer.sol

@@ -34,6 +34,8 @@ import {Panic} from "../Panic.sol";
  *     CircularBuffer.Bytes32CircularBuffer private myBuffer;
  * }
  * ```
+ *
+ * _Available since v5.1._
  */
 library CircularBuffer {
     /**

contracts/utils/structs/Heap.sol

@@ -33,6 +33,8 @@ import {StorageSlot} from "../StorageSlot.sol";
  * IMPORTANT: This library allows for the use of custom comparator functions. Given that manipulating
  * memory can lead to unexpected behavior. Consider verifying that the comparator does not manipulate
  * the Heap's state directly and that it follows the Solidity memory safety rules.
+ *
+ * _Available since v5.1._
  */
 library Heap {
     using Arrays for *;