Add "available since" comments in documentation

(cherry picked from commit 63a0343dda)

contracts/proxy/BeaconProxy.sol

@@ -11,6 +11,8 @@ import "./IBeacon.sol";
  *
  * The beacon address is stored in storage slot `uint256(keccak256('eip1967.proxy.beacon')) - 1`, so that it doesn't
  * conflict with the storage layout of the implementation behind the proxy.
+ *
+ * _Available since v3.4._
  */
 contract BeaconProxy is Proxy {
     /**

contracts/proxy/Clones.sol

@@ -12,6 +12,8 @@ pragma solidity >=0.6.0 <0.8.0;
  * The library includes functions to deploy a proxy using either `create` (traditional deployment) or `create2`
  * (salted deterministic deployment). It also includes functions to predict the addresses of clones deployed using the
  * deterministic method.
+ *
+ * _Available since v3.4._
  */
 library Clones {
     /**