Use System.Index in additional places

src/OpenIddict.Server.AspNetCore/OpenIddictServerAspNetCoreHandlers.cs

@@ -1095,7 +1095,7 @@ public static partial class OpenIddictServerAspNetCoreHandlers
             }
 
             // If the WWW-Authenticate header ends with a comma, remove it.
-            if (builder[builder.Length - 1] == ',')
+            if (builder[^1] == ',')
             {
                 builder.Remove(builder.Length - 1, 1);
             }

src/OpenIddict.Server.Owin/OpenIddictServerOwinHandlers.cs

@@ -1269,7 +1269,7 @@ public static partial class OpenIddictServerOwinHandlers
             }
 
             // If the WWW-Authenticate header ends with a comma, remove it.
-            if (builder[builder.Length - 1] == ',')
+            if (builder[^1] == ',')
             {
                 builder.Remove(builder.Length - 1, 1);
             }

src/OpenIddict.Validation.AspNetCore/OpenIddictValidationAspNetCoreHandlers.cs

@@ -558,7 +558,7 @@ public static partial class OpenIddictValidationAspNetCoreHandlers
             }
 
             // If the WWW-Authenticate header ends with a comma, remove it.
-            if (builder[builder.Length - 1] == ',')
+            if (builder[^1] == ',')
             {
                 builder.Remove(builder.Length - 1, 1);
             }

src/OpenIddict.Validation.Owin/OpenIddictValidationOwinHandlers.cs

@@ -700,7 +700,7 @@ public static partial class OpenIddictValidationOwinHandlers
             }
 
             // If the WWW-Authenticate header ends with a comma, remove it.
-            if (builder[builder.Length - 1] == ',')
+            if (builder[^1] == ',')
             {
                 builder.Remove(builder.Length - 1, 1);
             }