Update IAuthenticator? Authenticator in usage.md

[mavaddat]

Remove Authenticator from RestClient init to RestClientOptions in Usage docs.

Description

Update the Usage documentation so that it conforms to the latest RestSharp API regarding where to set the IAuthenticator? Authenticator.

New deprecation in RestSharp/RestClient.cs:

    [Obsolete("Use RestClientOptions.Authenticator instead")]
    public IAuthenticator? Authenticator => Options.Authenticator;

Deprecation warning CS0618: 'RestClient.Authenticator' is obsolete: 'Use RestClientOptions.Authenticator instead' in live:

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[mavaddat]

@dotnet-policy-service agree