Upgrade gulp to version 5.0.0

This is a major version bump, and the changelog at
https://github.com/gulpjs/gulp/releases/tag/v5.0.0 indicates one
breaking change that impacts us, namely that streams are now by default
interpreted/transformed to UTF-8 encoding. This breaks `gulp.src` calls
that work on binary files such as images or CMaps, but is fortunately
easy to fix for us by disabling re-encoding for all `gulp.src` calls
(see gulpjs/gulp#2764 (comment)
for more information). This restores the previous behavior of copying
the files as-is without Gulp performing any transformations to it, which
is what we want because Gulp is only used for bundling and we make sure
that the source files have the right encoding.