Support GitHub and Azure DevOps native auth methods

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Infrastrcuture as Code Type? (Required)

terraform

### Use Case (Required)

As a user of the accelerator, I want to use an alternative auth method than PAT for the accelerator bootstrap, e.g. gh cli or the standard env vars.

### Proposed Solution (Required)

Remove the input variable for the primary PAT as it is no longer required to be passed anywhere. Update the docs to show users how to auth with cli / env var instead.

### Important Factoids (Optional)

This was no possible in an older version, but is now due to requiring a specific lesser scoped PAT for agents / runners.

### References (Optional)

_No response_

Locate the Terraform accelerator bootstrap input for the primary PAT and the documentation that describes authentication. Start by tracing where that input is consumed, then review the existing GitHub CLI and environment-variable authentication paths. Done means the unused PAT input is removed and the documentation explains the supported native methods.

Written by the indexing model from the issue text.