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