[rust png] Mark `libpng` as not shipping in Chromium.

Since https://crrev.com/c/7108950 `libpng/visibility.gni` has been
trimmed, preventing `libpng` from being used outside of a handful of
tests.  This CL reflects this in `libpng/README.chromium` by explicitly
setting `Shipped: No` and `Security Critical: no`.

Bug: 443128323
Change-Id: Ic108cfea0851c5744e57a1f2a258a95301eacc53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7508066
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1572754}
NOKEYCHECK=True
GitOrigin-RevId: caf8fd17ea58c2b3804489501c4fa3950936d31d