fix: replace uses of obsolete method RNGCryptoServiceProvider (#766)

Replace insecure obsolete method (new RNGCryptoServiceProvider()) with RandomNumberGenerator.Create() in PkzipClassic, ZipFile and ZipOutputStream.

See docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.rngcryptoserviceprovider?view=net-6.0 and dotnet/runtime#40169

src/ICSharpCode.SharpZipLib/Encryption/PkzipClassic.cs

@@ -6,7 +6,7 @@ namespace ICSharpCode.SharpZipLib.Encryption
 {
 	/// <summary>
 	/// PkzipClassic embodies the classic or original encryption facilities used in Pkzip archives.
-	/// While it has been superceded by more recent and more powerful algorithms, its still in use and
+	/// While it has been superseded by more recent and more powerful algorithms, its still in use and
 	/// is viable for preventing casual snooping
 	/// </summary>
 	public abstract class PkzipClassic : SymmetricAlgorithm
@@ -444,7 +444,7 @@ public override byte[] Key
 		public override void GenerateKey()
 		{
 			key_ = new byte[12];
-			using (var rng = new RNGCryptoServiceProvider())
+			using (var rng = RandomNumberGenerator.Create())
 			{
 				rng.GetBytes(key_);
 			}

src/ICSharpCode.SharpZipLib/Zip/ZipFile.cs

@@ -3781,7 +3781,7 @@ private static void CheckClassicPassword(CryptoStream classicCryptoStream, ZipEn
 		private static void WriteEncryptionHeader(Stream stream, long crcValue)
 		{
 			byte[] cryptBuffer = new byte[ZipConstants.CryptoHeaderSize];
-			using (var rng = new RNGCryptoServiceProvider())
+			using (var rng = RandomNumberGenerator.Create())
 			{
 				rng.GetBytes(cryptBuffer);
 			}

src/ICSharpCode.SharpZipLib/Zip/ZipOutputStream.cs

@@ -723,7 +723,7 @@ private byte[] CreateZipCryptoHeader(long crcValue)
 			InitializeZipCryptoPassword(Password);
 
 			byte[] cryptBuffer = new byte[ZipConstants.CryptoHeaderSize];
-			using (var rng = new RNGCryptoServiceProvider())
+			using (var rng = RandomNumberGenerator.Create())
 			{
 				rng.GetBytes(cryptBuffer);
 			}
@@ -808,11 +808,11 @@ public override void Write(byte[] buffer, int offset, int count)
 
 		private void CopyAndEncrypt(byte[] buffer, int offset, int count)
 		{
-			const int CopyBufferSize = 4096;
-			byte[] localBuffer = new byte[CopyBufferSize];
+			const int copyBufferSize = 4096;
+			byte[] localBuffer = new byte[copyBufferSize];
 			while (count > 0)
 			{
-				int bufferCount = (count < CopyBufferSize) ? count : CopyBufferSize;
+				int bufferCount = (count < copyBufferSize) ? count : copyBufferSize;
 
 				Array.Copy(buffer, offset, localBuffer, 0, bufferCount);
 				EncryptBlock(localBuffer, 0, bufferCount);