Incorrect licenses

[sun]

Problem

• Our code is not GPL-2.0, but GPL-2.0-or-later.
• The license file does not indicate that it is a text file.

Proposed solution

1. Replace options GPL-2.0 and GPL-3.0 with GPL-2.0-or-later.
2. Download license document from Linux Foundation's license archive instead: https://spdx.org/licenses/
   • Or skip the downloading and commit two ready license files instead, GPL and MIT (alternatively ISC).
3. Rename template file to LICENSE.txt.

Notes

• The generator uses GitHub's license API currently, but https://api.github.com/licenses/GPL-2.0-or-later is not supported.
• The WordPress plugin developer documentation for the plugin header strongly suggests GPL-2.0-or-later: https://developer.wordpress.org/plugins/plugin-basics/header-requirements/

[sun]

https://www.gnu.org/licenses/old-licenses/#GPL also has the license documents and is the original source, of course. I don't know off-hand if there is a difference in the license text between 2.0 and 2.0+.