Use keepachangelog library into qgis-plugin-ci

[Gustry]

The dev behind https://github.com/Colin-b/keepachangelog is very active and has fixed all my requests.

He just released a version 2.0.0 beta 1. So I think we can start using it in qgis-plugin-ci.
I would keep the test of the changelog, only change the parser. It doesn't hurt to have test of external library.

@Guts I think your regexp is already nearly EOL :)

[Guts]

@Guts I think your regexp is already nearly EOL :)

😭

Really good news!