GH Actions: Update forked-helper and add some doc on secrets in workflow's README.md

[ColorfulRhino]

Description

::set-output has been deprecated in 2022: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
Thus replacing it with >> $GITHUB_OUTPUT as described in the guide above.

Also, took me a while to figure out how to create and use the ARMBIAN_SELF_DISPATCH_TOKEN mentioned in the readme. So I updated the readme accordingly so others don't have to spend time figuring it out on their own in the future.
(This helped me a lot: https://github.com/peter-evans/repository-dispatch?tab=readme-ov-file#token)

How Has This Been Tested?

• Ran the workflow on my forked repo without issues.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules