Ruby 4.0 integration has been fixed upstream (871d2cc). Remove the
hard-coded 3.4 pin in CI so we can use the latest Ruby version.

This reverts commit b8ebacc.

ci: Revert hard-coded Ruby 3.4 to use Ruby 4.0

The builtin iconv in macOS has been quite buggy since macOS 14, when
Apple replaced GNU iconv with a bespoke version. It introduced
backwards-incompatible changes, and behaves oddly in certain character
substitutions.

As such, build the official binary release using GNU iconv instead of
system iconv. This means we have to compile/cache it manually in our CI
just like gettext/libsodium in order to have a universal x86/arm64
binary with the correct deployment target set. We also need to modify
gettext to be built against GNU iconv as well to avoid link-time errors.

Note that this does not affect the Homebrew release of MacVim. The
standard Homebrew gettext is still linked against system iconv, and as
such we can't make an unilateral change without modifying Homebrew's
gettext as well.

This will result in the Vim binary being larger by 2 MB. It's not ideal
but tolerable. If Apple fixes their implementation of iconv we could
revert this in the future.

Related: #1624

Build MacVim binary release with GNU iconv instead of Apple iconv

Updated to Vim 9.1.2068