Rename FIRST_URL constant value from "$first" to "$top"

Updated the constant `FIRST_URL` in `RequestParser.cs` within the
`Azure.DataApiBuilder.Core.Parsers` namespace to use the value `"$top"`
instead of `"$first"`. This change aligns with naming conventions or
standards used elsewhere in the codebase or API, ensuring consistency
and improving clarity for developers.

Author: Mu4all

src/Core/Parsers/RequestParser.cs

@@ -30,7 +30,7 @@ public class RequestParser
         /// <summary>
         /// Prefix used for specifying limit in the query string of the URL.
         /// </summary>
-        public const string FIRST_URL = "$first";
+        public const string FIRST_URL = "$top";
         /// <summary>
         /// Prefix used for specifying paging in the query string of the URL.
         /// </summary>