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 */