Bump to lz4/lz4@5ff83968 [v1.9.4] (#7262)

Bump from lz4 v1.9.3 to v1.9.4.

Context: https://github.com/lz4/lz4/releases/tag/v1.9.4
Context: lz4/lz4@22e232d

Changes: lz4/lz4@d443718...5ff8396

	% git diff --shortstat d44371841a2f1728a3f36839fd4b7e872d0927d3...5ff839680134437dbf4678f3d0c7b371d84f4964
	 116 files changed, 7536 insertions(+), 2463 deletions(-)

The most important change from our point of view is:

  * faster decoding speed (~+20%) on aarch64 platforms

The release notes URL above mentions only Apple silicon platforms as
gaining from the improvement, but modern AArch64 chipsets may benefit
as well.

Author: grendello