Using those fields that have an underscore (instead of a hyphen) cause
the following warnings if used:
```
Warning: Input 'app_id' has been deprecated with message: 'app_id' is deprecated and will be removed in a future version. Use 'app-id' instead.
Warning: Input 'private_key' has been deprecated with message: 'private_key' is deprecated and will be removed in a future version. Use 'private-key' instead.
```

So this PR just drops the last use of `app_id` and `private_key` from
the README in favor of `app-id` and `private-key`.

GitHub's macOS runners for the past while have had some bad clock drift
which sometimes prevents this action from working with the error:

```console
'Issued at' claim ('iat') must be an Integer representing the time that the assertion was issued
```

`@octokit/auth-app` already has logic to handle this so we can defer to
that code.

## [1.6.2](v1.6.1...v1.6.2) (2023-12-06)

### Bug Fixes

* handle clock skew ([#87](#87)) ([495056a](495056a))