Change license to the GPL

[desrosj]

What?

This changes the license from the default one included in https://github.com/actions/javascript-action template to the GPL.

Why?

Free as in beer, free as in speech. 🍻

[github-actions]

Here is a list of everyone that appears to have contributed to this PR and any linked issues:

# Committers
Co-Authored-By: desrosj <[email protected]>

# Reviewers
Co-Authored-By: aaronjorbin <[email protected]>

# Commenters


# Reporters


# Unlinked

_{props-bot-action}

[aaronjorbin]

I think we need the header information about later versions of the GPL also working