License: GPL-2.0-or-later or GPL-3.0-or-Later

### What Happened?

The source files indicate a GPL-2.0-or-later license, but the actual license text is GPL-3.0 it would be good to clarify which is the appropriate license.

### Steps to Reproduce

Read:
- https://github.com/elementary/capnet-assist/blob/master/COPYING
- https://github.com/elementary/capnet-assist/blob/master/src/Application.vala
- https://github.com/elementary/capnet-assist/blob/master/src/MainWindow.vala
- https://github.com/elementary/capnet-assist/blob/master/src/TabbedWebView.vala

### Expected Behavior

License file type in repository should match that in code declaration.

### OS Version

Other Linux

### Session Type

Classic and Secure Sessions (I have tested in both)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

_No response_

Compare COPYING with the license declarations in src/Application.vala, src/MainWindow.vala, and src/TabbedWebView.vala. Start by reading those four files to identify the mismatch, then confirm with a maintainer which license is appropriate. Done means the repository license text and source declarations consistently state the agreed license.

Written by the indexing model from the issue text.