Start/Stop Runbook scripts should use Invoke-AzRestMethod to avoid retrieving a BearerToken

[SvenAelterman]

Description

As a TRE Administrator
I want to avoid getting a bearer token in TRE PS scripts
So that the code is easier to read and maintain.

Acceptance criteria

• Sample start and stop scripts at https://microsoft.github.io/AzureTRE/latest/tre-admins/start-stop/ use Invoke-AzRestMethod without explicitly the Bearer token for Invoke-RestMethod.
• Where possible, execute commands in parallel.

[SvenAelterman]

I created this issue as placeholder for myself. I will implement this for myself and after testing, create a PR.