Merge pull request #9100 from umbraco/netcore/task/6964-legacy-password-format

Adds support for the super old password format so we can handle upgrades

src/Umbraco.Infrastructure/BackOffice/BackOfficeUserStore.cs

@@ -246,6 +246,7 @@ namespace Umbraco.Core.BackOffice
             if (string.IsNullOrEmpty(passwordHash)) throw new ArgumentException("Value can't be empty.", nameof(passwordHash));
 
             user.PasswordHash = passwordHash;
+            user.PasswordConfig = null; // Clear this so that it's reset at the repository level
 
             return Task.CompletedTask;
         }