.NET v4.8: updating the Microsoft.Configuration.ConfigurationBuilders to version 3.0 does not work

[AlgorithmicThoughts]

I am working on a .NET 4.8 application, I updated the packages Microsoft.Configuration.ConfigurationBuilders (Environment, Json, UserSecrets) from version 2.0 to 3.0. I have also updated my app.config file to reflect the latest version, shown here:

<configBuilders>
    <builders>
      <add name="Environment" type="Microsoft.Configuration.ConfigurationBuilders.EnvironmentConfigBuilder, Microsoft.Configuration.ConfigurationBuilders.Environment, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
      <add name="Secrets" 
           userSecretsFile="./App_Data/secrets.xml" 
           mode="Greedy" 
           type="Microsoft.Configuration.ConfigurationBuilders.UserSecretsConfigBuilder, Microsoft.Configuration.ConfigurationBuilders.UserSecrets, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>      
    </builders>
  </configBuilders>

The structure of the secrets.xml file is given below:

<root>
<secrets ver="1.0">
    <secret name="App:UserKey" value="Hello World"/>
</secrets>
</root>

Before the update, when the version was 2.0. I was able to read the value from the secrets.xml file specified in the app.config key userSecretsFile:

var userKey = System.Configuration.ConfigurationManager.AppSettings["App:UserKey"];

After the update from version 2 to 3, this code no longer works and returns an empty string. I read the update guide but none of it provides troubleshooting guidance. I tried to update the path from ./App_Data/secrets.xml to ~/App_Data/secrets.xml for testing even then I get an empty string in the variable userKey.

Is there an additional step that needs to be performed in version 3 to make it work? Please suggest thanks