[FEAT]: Rename GITHUB_APP_PEM_FILE to GITHUB_APP_PRIVATE_KEY for consistency with GitHub terminology

### Describe the need

On the GitHub App configuration page (`https://github.com/organizations/[owner]/settings/apps/[app name]`) and throughout GitHub’s official documentation, the .pem file that a GitHub app provides is referred to as a "private key". The [GitHub Terraform provider](https://registry.terraform.io/providers/integrations/github/latest/docs), however, currently uses the variable name `GITHUB_APP_PEM_FILE` instead of `GITHUB_APP_PRIVATE_KEY`.

While functionally equivalent, this inconsistency makes knowledge transfer more difficult because:
1. Users must manually confirm that GITHUB_APP_PEM_FILE refers to the private key rather than another GitHub App credential.
2. Other GitHub tools, such as GitHub Actions (actions/create-github-app-token), use `private-key`, leading to unnecessary confusion.
3. Using GitHub’s native terminology improves clarity and reduces onboarding friction.

## Proposal

- Introduce GITHUB_APP_PRIVATE_KEY as the preferred variable name.
- Continue supporting GITHUB_APP_PEM_FILE as a deprecated alias for backward compatibility.

This change would ensure consistency across GitHub’s ecosystem while maintaining existing behavior for current users.

Could this improvement be considered?

### SDK Version

_No response_

### API Version

_No response_

### Relevant log output

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### Code of Conduct

- [x] I agree to follow this project's Code of Conduct