[macOS] Bump min. version to 10.13, and remove deprecated code.

[bruvzg]

Fixes godotengine/build-containers#124

Alternative to #76395.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.6.

[jasonliu--]

I realize that I'm necro-posting to an old thread, but I'd like to remind you guys that I have Godot 3.5.3 compiling and running all the way back to macOS 10.9 on MacPorts (yes, even the TPS Demo runs, extremely slowly, on my 10.9 virtual machine):

https://ports.macports.org/port/godot/details/

I am currently working on splitting off the Godot 3.x package into a separate godot3 package in MacPorts, but it would certainly be easier for me to maintain a Godot package that works for older macOSes if you didn't simply remove all of the old code, at least from the Godot 3.x branch.

In fact, I believe I'm somewhat close to getting Godot 4.x to compile and run on macOS all the way back to 10.11, which is the first version of macOS that has Metal, and thus there is the possibility of supporting Vulkan using MacPorts' MoltenVK package.