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
4 jobs for <span class="ref-name">wip/fmuellner/more-warnings</span> in 6 minutes and 43 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Source Check
passed #242861
eslint

00:00:23

 
  Build
passed #242862
flatpak

00:06:00

 
  Review
passed #242863
review

00:00:19

manual #242864
allowed to fail manual
stop_review