I'm working on "migrating" one of our Umbraco Cloud v8 sites over to v9 and I'm following the documentation here.

I have the new, blank v9 site running locally and I've also got the old v8 site running as well. I've copied the v8 database mdf files over to the umbraco\data folder in the v9 site and renamed them Umbraco8.mdf (.ldf). Now I'm trying to update the appsettings.json file's connection string as per the documentation above but I'm not familiar with how to properly specify the connection string to point to a particular mdf file.

Currently I have

"ConnectionStrings": {
    "umbracoDbDSN": "Data Source=(LocalDB)\\MSSQLLocalDB; AttachDbFileName=|DataDirectory|\\Umbraco8.mdf; Integrated Security=True; Connect Timeout=30; MultipleActiveResultSets=True"
},

but with this connection string the site doesn't go through the upgrade process like the documentation is telling me it should. Instead Umbraco is displaying a "Page Not Found" page and I'm pretty sure it's because I don't have the connection string correct.

I get the same behavior if I instead overwrite the v9 mdf files with the v8 mdf files and then clear out the connection string altogether.

Also, I'm not opposed to taking a backup of our v8 Development environment's database (bacpac) and restoring that into my localDB and then using a connection string to that db but when I tried to do that the import wizard failed with errors (see below).

    TITLE: Microsoft SQL Server Management Studio
------------------------------

Could not import package.
Warning SQL72012: The object [Heliosv8_Data] exists in the target, but it will not be dropped even though you selected the 'Generate drop statements for objects that are in the target database but that are not in the source' check box.
Warning SQL72012: The object [Heliosv8_Log] exists in the target, but it will not be dropped even though you selected the 'Generate drop statements for objects that are in the target database but that are not in the source' check box.
Error SQL72014: .Net SqlClient Data Provider: Msg 12824, Level 16, State 1, Line 5 The sp_configure value 'contained database authentication' must be set to 1 in order to alter a contained database.  You may need to use RECONFIGURE to set the value_in_use.
Error SQL72045: Script execution error.  The executed script:
IF EXISTS (SELECT 1
           FROM   [master].[dbo].[sysdatabases]
           WHERE  [name] = N'$(DatabaseName)')
    BEGIN
        ALTER DATABASE [$(DatabaseName)]
            SET CONTAINMENT = PARTIAL 
            WITH ROLLBACK IMMEDIATE;
    END


Error SQL72014: .Net SqlClient Data Provider: Msg 5069, Level 16, State 1, Line 5 ALTER DATABASE statement failed.
Error SQL72045: Script execution error.  The executed script:
IF EXISTS (SELECT 1
           FROM   [master].[dbo].[sysdatabases]
           WHERE  [name] = N'$(DatabaseName)')
    BEGIN
        ALTER DATABASE [$(DatabaseName)]
            SET CONTAINMENT = PARTIAL 
            WITH ROLLBACK IMMEDIATE;
    END


 (Microsoft.SqlServer.Dac)

How does one "Update the connection string in the Umbraco 9 AppSetting.Json file so that it connects to the Umbraco 8 database" as specified in the Umbraco Migrate Umbraco 8 Cloud project to Umbraco 9 documentation?