moving apidiff out of preview (#3595)

release-notes/3.0/api-diff/.Net/3.0.0_System.Net.md

@@ -0,0 +1,31 @@
+# System.Net
+
+``` diff
+ namespace System.Net {
+-    public class CookieCollection : ICollection, IEnumerable {
++    public class CookieCollection : ICollection, ICollection<Cookie>, IEnumerable, IEnumerable<Cookie>, IReadOnlyCollection<Cookie> {
++        public void Clear();
++        public bool Contains(Cookie cookie);
++        public bool Remove(Cookie cookie);
++        IEnumerator<Cookie> System.Collections.Generic.IEnumerable<System.Net.Cookie>.GetEnumerator();
+     }
+     public enum DecompressionMethods {
++        All = -1,
++        Brotli = 4,
+     }
+     public class FileWebRequest : WebRequest, ISerializable {
++        public override Task<Stream> GetRequestStreamAsync();
++        public override Task<WebResponse> GetResponseAsync();
+     }
+     public class IPEndPoint : EndPoint {
++        public static IPEndPoint Parse(ReadOnlySpan<char> s);
++        public static IPEndPoint Parse(string s);
++        public static bool TryParse(ReadOnlySpan<char> s, out IPEndPoint result);
++        public static bool TryParse(string s, out IPEndPoint result);
+     }
+     public enum SecurityProtocolType {
++        Tls13 = 12288,
+     }
+ }
+```
+