Switch add beatmap tag request to using path paramx

See ppy/osu-web#11999.

Author: bdach

osu.Game/Online/API/Requests/AddBeatmapTagRequest.cs

@@ -1,7 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-using System.Globalization;
 using System.Net.Http;
 using osu.Framework.IO.Network;
 
@@ -22,10 +21,9 @@ protected override WebRequest CreateWebRequest()
         {
             var req = base.CreateWebRequest();
             req.Method = HttpMethod.Post;
-            req.AddParameter(@"tag_id", TagID.ToString(CultureInfo.InvariantCulture), RequestParameterType.Query);
             return req;
         }
 
-        protected override string Target => $@"beatmaps/{BeatmapID}/tags";
+        protected override string Target => $@"beatmaps/{BeatmapID}/tags/{TagID}";
     }
 }