Add "available since" on Base64.sol

(cherry picked from commit bfd05d964604849b4282bb6a8337c8114da30826)

contracts/utils/Base64.sol

@@ -5,6 +5,8 @@ pragma solidity ^0.8.0;
 
 /**
  * @dev Provides a set of functions to operate with Base64 strings.
+ *
+ * _Available since v4.5._
  */
 library Base64 {
     /**