fix: Change embed description max length to 4096 (#1886)

* Update max embed description length (fixes #1881)

* Update unit tests for new embed builder length

src/Discord.Net.Core/Entities/Messages/EmbedBuilder.cs

@@ -27,7 +27,7 @@ namespace Discord
         /// <summary> 
         ///     Returns the maximum length of description allowed by Discord. 
         /// </summary>
-        public const int MaxDescriptionLength = 2048;
+        public const int MaxDescriptionLength = 4096;
         /// <summary> 
         ///     Returns the maximum length of total characters allowed by Discord. 
         /// </summary>

test/Discord.Net.Tests.Unit/EmbedBuilderTests.cs

@@ -126,7 +126,7 @@ namespace Discord
         {
             IEnumerable<string> GetInvalid()
             {
-                yield return new string('a', 2049);
+                yield return new string('a', 4097);
             }
             foreach (var description in GetInvalid())
             {
@@ -149,7 +149,7 @@ namespace Discord
             {
                 yield return string.Empty;
                 yield return null;
-                yield return new string('a', 2048);
+                yield return new string('a', 4096);
             }
             foreach (var description in GetValid())
             {