converters/libiconv: Update to 1.16

Update converters/libiconv to 1.16.
This update removes the extra patches for CP932 and EUCJP-MS, as they are
not compatible with this release.
Add an UPDATING entry for this change.
ABI/API stable, no need to bump dependent ports:
https://abi-laboratory.pro/index.php?view=timeline&l=libiconv

PR: 240954
Submitted by: Yuichiro NAITO
Exp-run by: antoine
Approved by: maintainer timeout (gnome@)