Set the resizable property in the Display section of the Project Settings to false by unchecking the checkbox.
However, I really don’t advise doing this, because non-resizable windows are not a good idea for people with unusual aspect ratios (such as 21:9) or resolutions. Ideally, your project should provide a GUI that scales to any realistic aspect ratio and resolution out there.
I would add borderless windows are mostly big functionality loss for little cosmetic gain. If your app fails at something, the window manager won’t expose any button to the user but those in the taskbar or the taskmanager. You will also likely loose system features like window anchoring, display selection or even moving, that you will have to re-implement yourself, if you can.
If you really want a borderless window, it should be smaller than any resolution you could encounter on desktop, be centered, and used only for a short splash screen (which Godot provides already?) or a small launcher.
If it’s not for that, don’t use borderless, or use full-screen.