U4-9812 FailedLoginAttempts should be reset to 0 after a successfull login attempt

(cherry picked from commit 3242b31a8a)

src/Umbraco.Core/Security/BackOfficeSignInManager.cs

@@ -191,6 +191,7 @@ namespace Umbraco.Core.Security
 
             //track the last login date
             user.LastLoginDateUtc = DateTime.UtcNow;
+            user.AccessFailedCount = 0;
             await UserManager.UpdateAsync(user);
 
             _logger.WriteCore(TraceEventType.Information, 0,
@@ -231,4 +232,4 @@ namespace Umbraco.Core.Security
             return null;
         }
     }
-}
+}