Add 'available since' notices

contracts/access/AccessControl.sol

@@ -59,6 +59,8 @@ abstract contract AccessControl is Context {
      *
      * `DEFAULT_ADMIN_ROLE` is the starting admin for all roles, despite
      * {RoleAdminChanged} not being emitted signaling this.
+     *
+     * _Available since v3.1._
      */
     event RoleAdminChanged(bytes32 indexed role, bytes32 indexed previousAdminRole, bytes32 indexed newAdminRole);