ffmpeg5: Update to ffmpeg-5.1.8 #785

Job	Run time
commit-message	2s
what-changed	26s
show-vars	2s
pkglint	44s
build-qemu (freebsd-14.2-x86_64, freebsd, x86-64, 14.2)	8m 4s
build-qemu (openbsd-7.6-x86_64, openbsd, x86-64, 7.6)	6m 36s
build-native (macos-15-arm64, macos-15)	6m 19s
build-native (cygwin-2022-x86_64, windows-2022)	1h 32m 11s
build-native (ubuntu-24.04-x86_64, ubuntu-24.04)	5m 34s
build-native (macos-13-x86_64, macos-13)	1s
jenkins / trigger	27m 39s
jenkins / build (centos-7-x86_64, https://ci.dreckly.dev/job/check-commit/check-commit=centos-7-x86_64/1064/)	2m 18s
jenkins / build (netbsd-10-x86_64, https://ci.dreckly.dev/job/check-commit/check-commit=netbsd-10-x86_64/10...	2m 15s
jenkins / build (smartos-24-x86_64, https://ci.dreckly.dev/job/check-commit/check-commit=smartos-24-x86_64/...	2m 16s
	2h 34m 27s

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