I'm going to play devil's advocate here for a moment, and throw some questions back to you.
Personally I'm against jQuery, since I'm a young developer, who wants to work in an up-to-date workflow.
So let's look at it from the other side. What reasons do you have for [whatever alternative]? You've mentioned: "I'm young", "an up-to-date workflow", "the possibilities with ES6", "personal development", and a "we want to move on".
How is this relevant?
Having a good, up-to-date workflow is probably your most reasonable argument. But then again, the workflow is not really that much related to which library you use or do not use. So, how is using jQuery precluding you from improving your workflow?
In a similar fashion, you can use ES6 and jQuery together, so this doesn't relate much to jQuery.
I'm going to guess that you're blaming "jQuery" for a larger absence of tools you want. We'll get to that in a moment but meanwhile, it would be a lot more useful if instead of generically blaming your problems on the particular use of jQuery you did a better job of analysis and better identified your problems with the current situation.
This is a fair concern but little more than fair. That is, it's important for you to seek personal development; it is fair and reasonable to expect that a job would provide you with opportunities to learn and grow technically and professionally. I insist, it's perfectly reasonable to expect this.
But then again, that should compromise neither the projects nor the company. That is, you should not expect the company to provide that "personal development" at the cost of the success of a project or the success of the company itself. And this is important because it means that you're a professional, and that you know that decisions about a project should be made taking into account a lot of different factors. Cost, in particular, is a very relevant one. It's of course important to invest resources in keeping a project reasonably up-to-date (previous point), but it's more important to keep it working and cost-effective. The concern of "personal development" is important, I insist, but relatively much less important than these others. What you're asking requires, quite probably, a noticeable investment, and you should -as a professional- consider if that can be balanced against "personal development" - maybe, but maybe not; I can't know, but you shouldn't assume either. You haven't explained why the insistence on using jQuery, You haven't explained if, maybe, this is because you're reusing code that is already built, tried and tested. I won't assume that, but since you haven't specified, I can't assume otherwise either.
Personal development, in any case, is something which can be achieved in various ways and it doesn't necessarily have to be done on all projects. You might have a much more reasonable argument if you proposed this as research, which is generally done on small, secondary, and/or non-critical projects. This way, you're not compromising -or not so much- important projects' success, and you have other arguments that support you, not only "personal development". I mean, you try some new tools, you evaluate them objectively and then you all compromise and accept the results of the evaluation. Did the use of X tool really improve something? If it did, you all agree to use it in more projects. If it didn't -or not enough-, well, at least you got your chance to try that new tool and work with it in a project.
Finally on this point, you seem to be assuming something wrong: That you cannot learn from tools that are not the latest and most up-to-date. If you do think that, you are extremely wrong. You can learn a lot.
There are three points to think about here. I'll leave out the one about "moving on", because it's mostly related to the things I have already mentioned above. But there's still two important points:
"want" is not an argument. You cannot present a professional argument to your boss or to anyone based on "because I want to". You do it twice above and a third one on a comment. Do not do it. It's not an argument.
But the point that most interests me here is this "we" you mention. Because, you see, up to that we, this all sounded mostly like a childish, selfish "boohoo, I want to play with the cool React thingy" (no offence meant, but this is what you sound like when your arguments are reduced to "I want" and "this is old, yuck"). But now you mention a "we". So, who is this "we"? Do others in the team agree with you? Who are these others and what are their arguments?
Now, I'm going to add a few more questions, not in response to what you said but to what you didn't say.
What are the actual requirements of the projects? i.e. you said "static sites", but fully static? What about browser support?
About that code reuse I mentioned, how is it? Do you actually reuse code from previous projects? Do you reuse tools, structures? Do you, maybe, have components -developed in-house or external- that you reuse throughout your projects? Something? Anything? Nothing at all?
And finally: Have you honestly, and in a non-confrontational manner, asked the person responsible about all this? Have you had a conversation with that person and talked about their motives?