Stop binary files being encoded with UTF-8

Gulp v5 changes the way that streams are encoded [1]:

> Our streams now default to UTF-8 encoding. Previously, our streams took whatever data was emitted without considering any encoding. However, in this release, we resolved a 10 year old issue to support custom encodings and we default these to UTF-8. Most usage of gulp won’t need to change anything, but certain plugins may produce non-UTF-8 output and you’ll need to set `{ encoding: false }` on your gulp streams.

As the fonts and images are binary we need to set `encoding: false` to avoid them being corrupted.

[1]: https://medium.com/gulpjs/announcing-gulp-v5-c67d077dbdb7#8f65

packages/govuk-frontend/tasks/assets.mjs

@@ -12,7 +12,8 @@ export const assets = (options) =>
       gulp
         .src('govuk/assets/**/*', {
           base: options.srcPath,
-          cwd: options.srcPath
+          cwd: options.srcPath,
+          encoding: false
         })
         .pipe(gulp.dest(options.destPath))
     )