CI: Pin to ffmpeg release 6.1 for Windows CI #426

Job	Run time
build-macos-no-brew-Qt6 (clang-9)	39s
build-macos-brew-Qt6 (clang-9)	1m 12s
build-linux-Qt6 (gcc-9)	8m 14s
build-linux (gcc-9)	7m 45s
build-macos-brew (clang-9)	2m 57s
build-macos-no-brew (clang-9)	1m 52s
build-windows-mingw (5.15.2)	9m 32s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	11m 28s
build-windows-vs (5.15.2)	8m 31s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	12m 50s
	1h 5m 0s

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