Add "available since" comments in documentation

(cherry picked from commit 63a0343dda)

contracts/presets/ERC20PresetFixedSupply.sol

@@ -12,6 +12,8 @@ import "../token/ERC20/ERC20Burnable.sol";
  *
  * This contract uses {ERC20Burnable} to include burn capabilities - head to
  * its documentation for details.
+ *
+ * _Available since v3.4._
  */
 contract ERC20PresetFixedSupply is ERC20Burnable {
     /**

contracts/presets/ERC777PresetFixedSupply.sol

@@ -8,6 +8,8 @@ import "../token/ERC777/ERC777.sol";
  *
  *  - Preminted initial supply
  *  - No access control mechanism (for minting/pausing) and hence no governance
+ *
+ * _Available since v3.4._
  */
 contract ERC777PresetFixedSupply is ERC777 {
     /**