Backport pull request #15752 from jellyfin/release-10.11.z

Fix case sensitivity edge case

Original-merge: b50ce1ad6b3239245897a648dd24cf407138abfc

Merged-by: Bond-009 <bond.009@outlook.com>

Backported-by: Bond_009 <bond.009@outlook.com>

Jellyfin.Server.Implementations/Users/UserManager.cs

@@ -149,7 +149,7 @@ namespace Jellyfin.Server.Implementations.Users
 
             ThrowIfInvalidUsername(newName);
 
-            if (user.Username.Equals(newName, StringComparison.OrdinalIgnoreCase))
+            if (user.Username.Equals(newName, StringComparison.Ordinal))
             {
                 throw new ArgumentException("The new and old names must be different.");
             }