Upgrade gulp to version 5.0.0

[timvandermeij]

We should upgrade to Gulp 5 now that it's officially released. The changelog, including the breaking changes that mostly don't seem to affect us, can be found at https://github.com/gulpjs/gulp/releases/tag/v5.0.0.

I have already quickly tried the upgrade locally to see if it would be easy or not, and found that the main blocker appears to be the merge-stream package. It doesn't seem to work anymore with Gulp 5, causing tracebacks for most tasks (because of most of them directly or indirectly rely on merge calls).

The last commit for merge-stream, see https://github.com/grncdr/merge-stream, was 5 years ago, so this probably isn't very surprising. However, I played around with removing the merge calls altogether and didn't immediately find something that was obviously broken, so I'm mainly wondering what the use case of it is or was at the time of introduction (maybe it's obsolete nowadays, or something breaks when removing it that I just haven't noticed yet...). Sadly I haven't been able to find out more from e.g. Git commit messages.

We should investigate if merge-stream can be fixed, replaced or removed. Once that is addressed Gulp 5 should work, but note that we still need to carefully test all commands.

[Snuffleupagus]

Could https://github.com/sindresorhus/merge-streams be a possible replacement?