Update fa-brands-400.* files to more recent versions

[kghbln]

I am unsure which version of FA the current files were taken from; however, updated versions for FA 5.15.4 or 6.5.2 are available. I found the 5.15.4 versions here, but this is only a secondary source. I don't know how to get the combined brands file from FA directly. Anyhow, it will be cool to update these since people apparently start to dearly miss the X logo.

[kghbln]

There may be even more work involved to make e.g. X happen. If so, it will be good to address it with this issue, too.