Fixes #24722: Update gulp related js dependencies

[RaphaelGauthier]

https://issues.rudder.io/issues/24722

Only two js dependencies have been updated in this PR :

• del : from 6.1.1 to 7.1.0
• gulp : from 4.0.2 to 5.0.0

The gulpfile.js file (now gulpfile.mjs) has been converted into an Ecmascript module , so that the new version of del package can be imported as a module (that package has become an esm-only package in this new version). gulp-cli supports ESM so this change has no side effect.

This implies a few changes to the file :

• Most of them are listed in this guide : https://gist.github.com/noraj/007a943dc781dc8dd3198a29205bae04
• clean() function has been rewritten because of the change in the behaviour of the del package functions
• and in addition, merge-stream has been replaced by merge2 as it did not work with the new changes, and gulp-sourcemaps has been downgraded to the 2.6.5 version to avoid vulnerabilities present in the 3.0.0 version.

[RaphaelGauthier]

Do not merge yet, I'm having issues with fontawesome and OTS errors :

[RaphaelGauthier]

PR updated with a new commit

[RaphaelGauthier]

I had to add this option to make ionicons and fontawesome work.

Gulp 5 stream encoding is now set to UTF-8 by default, which is not compatible with these icon libs :

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.

[RaphaelGauthier]

Do not merge yet, I'm having issues with fontawesome and OTS errors :

It's now working, merge can be done !

[RaphaelGauthier]

PR rebased

[Normation-Quality-Assistant]

This PR is not mergeable to upper versions.
Since it is "Ready for merge" you must merge it by yourself using the following command:
rudder-dev merge https://github.com/Normation/rudder/pull/5606
-- Your faithful QA
Kant merge: "It is beyond a doubt that all our knowledge begins with experience."
(https://ci.normation.com/jenkins/job/merge-accepted-pr/85599/console)