cleanup: Rename all .css files to .qss. #1687

Job	Run time
Validate PR	8s
Check for translatable strings	19s
Verify release asset signatures	8s
Update nightly release tag	5s
Clang-Tidy	6m 51s
Alpine (full, Debug)	3m 53s
Alpine (static) (Release)	3m 2s
Debian (minimal, Debug)	2m 28s
Fedora with ASAN (full, Debug)	3m 32s
Ubuntu LTS (full, Release)	3m 43s
macOS user (x86_64)	6m 5s
AppImage on alpine (full, Release)	3m 8s
Android (arm64-v8a, Release, 6.2.4)	3m 25s
Flatpak	4m 36s
Docs	1m 49s
macOS distributable (arm64)	2m 41s
Windows (x86_64, Release)	4m 48s
Test macOS distributable (arm64)	9s
Test AppImage	25s
Test Flatpak	51s
	52m 6s

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