build: bump FFmpeg from "gpl 7.0" to "gpl 7.1" #30

Job	Run time
Build base image	8m 35s
Build target base image (win64)	46m 4s
Build target base image (linux64)	43m 46s
Build target base image (linuxarm64)	43m 48s
Build target-variant image (win64, gpl 7.1)	1h 8m 1s
Build target-variant image (linux64, gpl 7.1)	59m 15s
Build target-variant image (linuxarm64, gpl 7.1)	55m 54s
Build ffmpeg (win64, gpl 7.1)	18m 9s
Build ffmpeg (linux64, gpl 7.1)	15m 26s
Build ffmpeg (linuxarm64, gpl 7.1)	14m 53s
Publish release	43s
	6h 14m 34s

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