Add "available since" comments in documentation

(cherry picked from commit 63a0343dda)

contracts/drafts/EIP712.sol

@@ -18,6 +18,8 @@ pragma solidity >=0.6.0 <0.8.0;
  *
  * NOTE: This contract implements the version of the encoding known as "v4", as implemented by the JSON RPC method
  * https://docs.metamask.io/guide/signing-data.html[`eth_signTypedDataV4` in MetaMask].
+ *
+ * _Available since v3.4._
  */
 abstract contract EIP712 {
     /* solhint-disable var-name-mixedcase */

contracts/drafts/ERC20Permit.sol

@@ -15,6 +15,8 @@ import "./EIP712.sol";
  * Adds the {permit} method, which can be used to change an account's ERC20 allowance (see {IERC20-allowance}) by
  * presenting a message signed by the account. By not relying on `{IERC20-approve}`, the token holder account doesn't
  * need to send a transaction, and thus is not required to hold Ether at all.
+ *
+ * _Available since v3.4._
  */
 abstract contract ERC20Permit is ERC20, IERC20Permit, EIP712 {
     using Counters for Counters.Counter;