licence: Replace with Markdown version

Replaced LICENSE.md with official Markdown GPL from
https://www.gnu.org/licenses/gpl-3.0.md
2 jobs for FrostyPenguin-master-fix-licence-markdown in 4 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  .Pre
passed #592375
flatpak

00:04:41

passed #592376
rustfmt

00:00:46