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>

Documentation/git-update-index.txt

@@ -166,9 +166,11 @@ you will need to handle the situation manually.
 +
 Version 4 performs a simple pathname compression that reduces index
 size by 30%-50% on large repositories, which results in faster load
-time. Version 4 is relatively young (first released in 1.8.0 in
-October 2012). Other Git implementations such as JGit and libgit2
-may not support it yet.
+time.  Git supports it since version 1.8.0, released in October 2012,
+and support for it was added to libgit2 in 2016 and to JGit in 2020.
+Older versions of this manual page called it "relatively young", but
+it should be considered mature technology these days.
+
 
 -z::
 	Only meaningful with `--stdin` or `--index-info`; paths are