Replace usage of the `set-output` command in the `.github/scripts/process_user_data_file.py` helper script

# 💡 Summary #

The `set-output` GitHub Actions command is currently used here:
https://github.com/cisagov/windows-commando-vm-packer/blob/99b30f5f520a76f64acbde7c9357829ff54453b9/.github/scripts/process_user_data_file.py#L23

It should be replaced with a supported method of setting a GitHub Action output.

## Motivation and context ##

The `set-output` command has been deprecated by GitHub per:
- https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands
- https://github.blog/changelog/2023-07-24-github-actions-update-on-save-state-and-set-output-commands

## Implementation notes ##

GitHub offers a replacement method to achieve the same end result.

## Acceptance criteria ##

- [ ] Usage of `set-output` has been replaced with a supported method.

Open .github/scripts/process_user_data_file.py and inspect the set-output usage at line 23. Read the linked GitHub Actions deprecation guidance to identify the supported replacement, then verify the helper no longer uses set-output and that its output is still available to the workflow.

Written by the indexing model from the issue text.