update-index doc: v4 is OK with JGit and libgit2

update-index doc: v4 is OK with JGit and libgit2

Being invented in late 2012 no longer makes the index v4 format
"relatively young".

The support for the index version 4 was added to libgit2 with their
5625d86b (index: support index v4, 2016-05-17) and to JGit with
their e9cb0a8e (DirCache: support index V4, 2020-08-10).

Let's update the paragraph that discouraged its use for folks overly
cautious about cross-tool compatibility.

Helped-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Helped-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>