How do you deal with management and tech debt?

Hey all, I want to know how you all deal with management and pushing tech debt work. Here's a little bit of background on my current situation, and I'd love to hear how you'd deal with it.

I've been in the profession for about 8 years and had a high-level job at my last company where I oversaw a huge amount of modernization work (bringing an old Laravel codebase up to PHP 8, putting all sites in Docker images for the new cloud infrastructure etc...).

I recently got a new remote job with a pretty high salary (I swear this is relevant and not a brag) with a company that has an ancient tech stack. During the interview, we talked about modernizing the company's stack and seemed to be quite important to them. I really like the company and the people working there and I've been really welcomed there. I was brought into the role because of my experience with modernizing code and I worked for a competitor before joining this team.

The tech stack here is pretty simple and ancient. It does work, but it causes a lot of issues. They're using a monolithic Apache server for all of the websites we manage which each dev has to set up with virtual hosts. My first main project is working under a senior dev to scope out a brand new Laravel API which is all modern tech, no outdated PHP versions or anything.

I was pretty pumped the past few weeks but today I hit a lot of roadblocks in working with him and kind of want to hear what you guys feel about the situation.

We're building out an API specification and he insisted that we do it in a Google document, which I suggested we look at an OpenAPI specification instead so we didn't have to keep repeating request bodies and responses. He came back and said something along the lines of: "I don't really want to learn YAML because I don't have time, so we'll stick with the document.". My wrists and fingers still ache from having to copy, paste and edit each request and response manually. Google Docs isn't a great solution for generating API specifications.

Then after that, we bootstrapped the main Laravel application. It's the most recent version of Laravel, and I realised that he'd committed the whole vendor folder to the repo and had gone through the .gitignore files in each dependency and removed stuff that would mess with it. I asked why he did it like that, and he said: "we won't be using Composer because our servers don't have it". Our other applications are running on an older version of PHP so I said we'd need a new server anyways, so why don't we do it the way that Laravel suggests with CI/CD pipelines? He comes back and says "We don't use Composer, and that won't change.". He's been pretty cold to me ever since I started.

Thanks for sticking with me, now back to the salary. How should I approach my manager (the Lead Developer) about this without making it seem like I'm tattling on the Senior? The salary is way more than an average Laravel dev and I know I'll feel bad if I say nothing. I also don't want to dull my skills with newer technologies because I'll struggle in my next role when/if I move on. I spent 3/4 years at my last role and then moved onto another role which only lasted 3 months before coming into this role, so I don't really want to change jobs again for a while.

I'd really value your opinions in this as professionals, even if the technology I've mentioned isn't familiar to you! How would you deal with this situation, especially when it comes to management that don't understand the problems that ignoring tech debt can cause?