Upgrade to libiconv 1.16

[flavorjones]

libiconv v1.16 was released in 2019. Nokogiri v1.8.0..v1.11.2 has been using libiconv v1.15.

Looking at the Changelog, there doesn't seem to be an urgent reason to upgrade, but we should generally try to stay current.

Note that libiconv is only used for precompiled libraries on these platforms:

• macOS ("x86_64-darwin" and "arm64-darwin")
• Windows ("x86-mingw32" and "x64-mingw32")

See LICENSE-DEPENDENCIES.md for more information.

Punchlist:

• update the dependencies.yml file
• test a Windows native gem built with it
• test a macOS native gem built with it

See b788241 for prior update to v1.15, to see what changes.