Update usage of 'Get-AzAccessToken' due to upcoming breaking changes in the cmdlet

[NowinskiK]

Is your feature request related to a problem? Please describe. The code uses Get-AzAccessToken cmdlet from Az.Accounts. For example, run Get-AdfFromService and you'll see the following warning:

WARNING: Upcoming breaking changes in the cmdlet 'Get-AzAccessToken' :
The Token property of the output type will be changed from String to SecureString. Add the [-AsSecureString] switch to avoid the impact of this upcoming breaking change.
- The change is expected to take effect in Az version : '13.0.0'
- The change is expected to take effect in Az.Accounts version : '4.0.0'
Note : Go to https://aka.ms/azps-changewarnings for steps to suppress this breaking change warning, and other information on breaking changes in Azure PowerShell.

When a user updates 'Az.Accounts' to version 4.0, the whole library stops working.

Describe the solution you'd like Follow the instructions (warning) given above and update code which uses Get-AzAccessToken cmdlet.