chore: Upgrade to kde platform 6.8 in flatpak. #512

Job	Run time
build (alpine-s390x)	13s
buildfarm (server)	14s
build (bazel)	3m 9s
qtox-host-qt (6.2.4)	14s
buildfarm (worker)	11s
build (compcert)	22s
qtox-host-qt (6.8.1)	16s
build (doxygen)	20s
build (flutter-web)	13s
build (frama-c)	15s
build (freebsd)	13s
build (haskell)	19s
build (pkgsrc)	15s
qtox-android (armeabi-v7a, debug, 6.2.4)	29s
qtox-android (armeabi-v7a, debug, 6.8.1)	15s
qtox (alpine)	13s
qtox-android (armeabi-v7a, release, 6.2.4)	22s
qtox (alpine-appimage)	15s
qtox-android (armeabi-v7a, release, 6.8.1)	15s
qtox (alpine-static)	22s
qtox-android (arm64-v8a, debug, 6.2.4)	15s
qtox (debian)	14s
qtox-android (arm64-v8a, debug, 6.8.1)	21s
qtox (fedora)	22s
qtox-android (arm64-v8a, release, 6.2.4)	16s
qtox (flatpak-builder)	1m 40s
qtox-android (arm64-v8a, release, 6.8.1)	25s
qtox (ubuntu-lts)	16s
qtox (windows-builder.i686)	19s
qtox (windows-builder.x86_64)	22s
	12m 55s

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