Remove all 'available since' notices

contracts/math/SafeMath.sol

@@ -51,8 +51,6 @@ library SafeMath {
      *
      * Requirements:
      * - Subtraction cannot overflow.
-     *
-     * _Available since v2.4.0._
      */
     function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
         require(b <= a, errorMessage);
@@ -109,8 +107,6 @@ library SafeMath {
      *
      * Requirements:
      * - The divisor cannot be zero.
-     *
-     * _Available since v2.4.0._
      */
     function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
         // Solidity only automatically asserts when dividing by 0
@@ -146,8 +142,6 @@ library SafeMath {
      *
      * Requirements:
      * - The divisor cannot be zero.
-     *
-     * _Available since v2.4.0._
      */
     function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
         require(b != 0, errorMessage);

contracts/token/ERC721/ERC721Metadata.sol

@@ -93,8 +93,6 @@ contract ERC721Metadata is Context, ERC165, ERC721, IERC721Metadata {
     /**
      * @dev Internal function to set the base URI for all token IDs. It is
      * automatically added as a prefix to the value returned in {tokenURI}.
-     *
-     * _Available since v2.5.0._
      */
     function _setBaseURI(string memory baseURI) internal virtual {
         _baseURI = baseURI;
@@ -104,8 +102,6 @@ contract ERC721Metadata is Context, ERC165, ERC721, IERC721Metadata {
     * @dev Returns the base URI set via {_setBaseURI}. This will be
     * automatically added as a preffix in {tokenURI} to each token's URI, when
     * they are non-empty.
-    *
-    * _Available since v2.5.0._
     */
     function baseURI() external view returns (string memory) {
         return _baseURI;

contracts/utils/Address.sol

@@ -47,8 +47,6 @@ library Address {
      * taken to not create reentrancy vulnerabilities. Consider using
      * {ReentrancyGuard} or the
      * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
-     *
-     * _Available since v2.4.0._
      */
     function sendValue(address payable recipient, uint256 amount) internal {
         require(address(this).balance >= amount, "Address: insufficient balance");

contracts/utils/Create2.sol

@@ -8,8 +8,6 @@ pragma solidity ^0.6.0;
  *
  * See the https://eips.ethereum.org/EIPS/eip-1014#motivation[EIP] for more
  * information.
- *
- * _Available since v2.5.0._
  */
 library Create2 {
     /**

contracts/utils/EnumerableSet.sol

@@ -15,8 +15,6 @@ pragma solidity ^0.6.0;
  *
  * Include with `using EnumerableSet for EnumerableSet.AddressSet;`.
  *
- * _Available since v2.5.0._
- *
  * @author Alberto Cuesta Cañada
  */
 library EnumerableSet {

contracts/utils/ReentrancyGuard.sol

@@ -15,9 +15,6 @@ pragma solidity ^0.6.0;
  * TIP: If you would like to learn more about reentrancy and alternative ways
  * to protect against it, check out our blog post
  * https://blog.openzeppelin.com/reentrancy-after-istanbul/[Reentrancy After Istanbul].
- *
- * _Since v2.5.0:_ this module is now much more gas efficient, given net gas
- * metering changes introduced in the Istanbul hardfork.
  */
 contract ReentrancyGuard {
     bool private _notEntered;

contracts/utils/SafeCast.sol

@@ -15,8 +15,6 @@ pragma solidity ^0.6.0;
  *
  * Can be combined with {SafeMath} to extend it to smaller types, by performing
  * all math on `uint256` and then downcasting.
- *
- * _Available since v2.5.0._
  */
 library SafeCast {