ci: Bump the warning-level

With the previous fixes, we can now build with an increased warning
level to catch more errors. Unfortunately we cannot use the highest
level, as the platform itself relies on some features that -Wpedantic
complains about.

!99
2 jobs from master in 7 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Source Check
passed #242873
eslint

00:00:29

 
  Build
passed #242874
flatpak

00:06:44