Update copyright to incorporate AGPL3.0+

[pljones]

Short description of changes

Update copyright on all existing files to GPL3.0+ and add a notice that all new contributions must be licensed under AGPL3.0+.

Initial changes made by Cursor Composer 2.5.

• Diffs checked manually for consistency with intended changes.
• Files list checked manually for consistency with intended changes.
• Manually added headers and corrected dates where they looked wrong (legacy issue).

CHANGELOG: Update copyright to incorporate AGPL3.0+

Context: Fixes an issue?

Fixes: 3711

Does this change need documentation? What needs to be documented and how?

No - documented by the issue.

Status of this Pull Request

Hopefully there's nothing changed in the code or build here...

What is missing until this pull request can be merged?

Someone needs to read what I wrote in various places until bored...

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above

AUTOBUILD: Please build all targets

[ann0see]

I've added some more users which are part of the organization and should - if they have thoughts on this respond.

[pljones]

• I tested my code and it does what I want

I am not a lawyer and I hope we never have to test the changes, to be honest.

I've also not checked the signing isn't broken... @ann0see -- I'd need to merge this on your tree, I think, before the Auto-Build there will do the signing. Is there any other way?

[ann0see]

• I'd need to merge this on your tree, I think, before the Auto-Build there will do the signing. Is there any other way?

Feel free to push it even as the latest release. The repo is safe to experiment with and nobody should be using the binaries from there anyway (except for us copying the signed build)