I admit that I feel for the senior dev in this story.
I've been in this situation before, you're stuck maintaining a combination of older systems, and you need to add another one with some new team-members. It's going to have the latest technologies like Angular / Beanstalk / Webpack, etc... Then the new guy quits / gets into an argument / doesn't make it through probation, etc. and now you as the senior dev are stuck maintaining a raw PHP 5 / PHP 7 / PHP 8 / Angular / Beanstalk / Docker combination. Let's not talk about Laravel's custom build environment that they've been pushing for a while that basically no one seems to use. I've come to especially dislike CI/CD systems as not only are they flaky and a pain to set up, but I've also seen people get locked out of the management permissions and then I'm stuck doing keyhole surgery to triangulate issues. As someone still on their probation, the senior dev probably has some concerns with letting you give suggestions regarding the tech stack, once it's clear you're going to stick around then your suggestions would have a lot more weight.
Asks yourself, is this an issue worth picking a fight over? Is composer so critical that you're willing to lose your job over it? What about OpenAPI, are you willing to give up your job over not having it? I think it's worth taking a step back and re-assessing, IT will always have word salad new technologies, they come and go, but they don't really change all that much about the project so I wouldn't get too attached to them.
You as a probationary dev, should absolutely not under any circumstances bad mouth your senior to the lead, given that you're new they have no reason to take your word while your senior will have completed projects under their belt. The only thing that it can do is make you look unreliable to the management.