Actually apply the GNU licenses

[LilithHafner]

The GNU licenses contain instructions for applying those licenses to works. They say that, ideally, folks should add a notice to each file. However, the template as was did not actually state that the works are licensed under the included license. This PR changes that by copying the recommended verbiage from each respective license.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (564c452) 92.95% compared to head (86c360c) 92.95%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #447 +/- ## ======================================= Coverage 92.95% 92.95% ======================================= Files 24 24 Lines 738 738 ======================================= Hits 686 686 Misses 52 52 ```

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[LilithHafner]

For GPL-3.0+ I got the text from

https://github.com/JuliaCI/PkgTemplates.jl/blob/46997ed6b70dd988fcaccadfc7746929f41e4cee/templates/licenses/GPL-3.0%2B#L635-L648

For GPL-2.0+ I got the text from

https://github.com/JuliaCI/PkgTemplates.jl/blob/46997ed6b70dd988fcaccadfc7746929f41e4cee/templates/licenses/GPL-2.0%2B#L294-L308

For LGPL-3.0+ I got the text from

https://github.com/JuliaCI/PkgTemplates.jl/blob/46997ed6b70dd988fcaccadfc7746929f41e4cee/templates/licenses/GPL-3.0%2B#L635-L648 with slight modification.

For LGPL-2.1+ I got the text from

https://github.com/JuliaCI/PkgTemplates.jl/blob/46997ed6b70dd988fcaccadfc7746929f41e4cee/templates/licenses/LGPL-2.1%2B#L474-L489

For AGPL3.0+ I got the text from

https://github.com/JuliaCI/PkgTemplates.jl/blob/46997ed6b70dd988fcaccadfc7746929f41e4cee/templates/licenses/AGPL-3.0%2B#L633-L646

[gdalle]

Thanks!