chore: Add support for macOS 10.15 with Qt 6.6.3. #564

Job	Run time
buildfarm (server)	22s
buildfarm (worker)	22s
build (alpine-s390x)	24s
qtox-host-qt (6.2.4)	19s
build (bazel)	3m 59s
qtox-host-qt (6.8.1)	19s
build (compcert)	25s
build (doxygen)	24s
build (flutter-web)	24s
build (frama-c)	17s
build (freebsd)	33m 5s
build (haskell)	18s
build (pkgsrc)	16s
qtox (alpine)	16s
qtox-android (armeabi-v7a, debug, 6.2.4)	19s
qtox-android (armeabi-v7a, debug, 6.8.1)	16s
qtox (alpine-appimage)	22s
qtox-android (armeabi-v7a, release, 6.2.4)	20s
qtox-android (armeabi-v7a, release, 6.8.1)	16s
qtox (alpine-static)	22s
qtox-android (arm64-v8a, debug, 6.2.4)	26s
qtox (debian)	22s
qtox-android (arm64-v8a, debug, 6.8.1)	16s
qtox (fedora)	17s
qtox-android (arm64-v8a, release, 6.2.4)	26s
qtox (flatpak-builder)	14s
qtox-android (arm64-v8a, release, 6.8.1)	27s
qtox (ubuntu-lts)	22s
qtox (windows-builder.i686)	23s
qtox (windows-builder.x86_64)	13s
	46m 31s

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