Asklemmy

"This is my first IT job, I’ve only been working here 3 months"

Then you need to learn this lesson quickly: YOU ARE NOT THE BOSS. The Boss is the Boss. Not you. You make your concerns known to him then you leave it at that.

"I’m considering talking directly to the owners about this issue" Yeah, going over his head is really going to go down well /s. As you have proven you are hard of learning, let me state clearly: it won't, that was sarcasm. The owners will see you've gone over your boss's head and when he says "I've had enough of this jerk, let's get someone else in" they'll be hard pressed to disagree with him.

"my boss’s refusal puts our operations at risk" Your boss already knows this. Especially as you keep banging on about it. What you're doing here is heading for an unceremonious out-kicking. Your boss also knows a lot more about the business than you do. If he's keeping that machine on Win7 then he probably has some good reasons to do so.

"I want to ensure I handle this professionally" No you don't. You want to force your boss to do what you think he should do. If you were being professional you'd state your concerns, in email if necessary, then move on.

"I definitely feel like I’m going to be used as a scapegoat" That's why you put your concerns in an email (ONLY to your boss, nobody else. Or maybe a sympathetic team member). This creates a paper trail so that if and when they come knocking on your door saying "Why did you let this happen! You're fired!" you can point to that email which proves you did everything you could. (Which they won't by the way. You're an idiot newb three months into your first job. You don't have any responsibility yet. So this isn't on you.)

"I’m also planning on seeking employment elsewhere" It doesn't matter where you work while you have this attitude. Newsflash kiddo: you're the asshole here. You're a newb three months into your first job. No matter what you think you know, you don't know anything. Instead of trying to dictate to others what you think they should do, try to learn why they're doing it differently from what you expect. Maybe you have to find somewhere else now; that boat may have already sailed. Maybe if you approach your boss saying something like "er, sorry I was an asshole, I thought I knew more than I do, can we start over and I want to learn from you" (but obvs phrase it better than that) then MAYBE you stand a chance of getting through your first year.

[Sympathetic mode on.]

We all have to learn this stuff and it takes time. Your boss also knows this, and remembers when he was an overenthusiastic hothead. So while all the above might seem harsh, especially the YTA bit, hopefully it'll cause a course correction (which is my intent here) and you'll be back on track to a successful career in IT. This position may still be salvageable but you need to go in on Monday understanding clearly that it might not be, and that it is your fault. And maybe you need to be fired a few times before this sinks in. Good luck.