remove beta notice

contracts/GSN/README.adoc

@@ -15,8 +15,6 @@ Utilities to make writing xref:ROOT:gsn-strategies.adoc[GSN strategies] easy are
 
 You can also take a look at the two contract interfaces that make up the GSN protocol: {IRelayRecipient} and {IRelayHub}, but you won't need to use those directly.
 
-NOTE: This feature is being released in the next version of OpenZeppelin Contracts, available right now through `npm install @openzeppelin/contracts@next`.
-
 == Recipient
 
 {{GSNRecipient}}

contracts/math/SafeMath.sol

@@ -52,9 +52,6 @@ library SafeMath {
      * Requirements:
      * - Subtraction cannot overflow.
      *
-     * NOTE: This is a feature of the next version of OpenZeppelin Contracts.
-     * @dev Get it via `npm install @openzeppelin/contracts@next`.
-     *
      * _Available since v2.4.0._
      */
     function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
@@ -112,9 +109,6 @@ library SafeMath {
      *
      * Requirements:
      * - The divisor cannot be zero.
-
-     * NOTE: This is a feature of the next version of OpenZeppelin Contracts.
-     * @dev Get it via `npm install @openzeppelin/contracts@next`.
      *
      * _Available since v2.4.0._
      */
@@ -153,9 +147,6 @@ library SafeMath {
      * Requirements:
      * - The divisor cannot be zero.
      *
-     * NOTE: This is a feature of the next version of OpenZeppelin Contracts.
-     * @dev Get it via `npm install @openzeppelin/contracts@next`.
-     *
      * _Available since v2.4.0._
      */
     function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {

contracts/utils/Address.sol

@@ -34,9 +34,6 @@ library Address {
      * @dev Converts an `address` into `address payable`. Note that this is
      * simply a type cast: the actual underlying value is not changed.
      *
-     * NOTE: This is a feature of the next version of OpenZeppelin Contracts.
-     * @dev Get it via `npm install @openzeppelin/contracts@next`.
-     *
      * _Available since v2.4.0._
      */
     function toPayable(address account) internal pure returns (address payable) {

docs/modules/ROOT/pages/gsn.adoc

@@ -4,8 +4,6 @@ The https://gsn.openzeppelin.com[Gas Station Network] allows you to build apps w
 
 If you're new to the GSN, you probably want to first take a look at the xref:openzeppelin::gsn/what-is-the-gsn.adoc[light overview of the system], to get a clearer picture of how gasless transactions are achieved. Otherwise, strap in!
 
-NOTE: This feature is being released in the next version of `@openzeppelin/contracts`, available right now through `npm install @openzeppelin/contracts@next`. It is also available on the `@openzeppelin/contracts-ethereum-package` variant.
-
 == Receiving a relayed call
 
 The first step to writing a recipient is to inherit from our GSNRecipient contract. If you're also inheriting from other OpenZeppelin contracts, such as ERC20 or Crowdsale, this will work just fine: adding GSNRecipient to all of your token or crowdsale functions will make them GSN-callable.