Deprecate the pointless unsafe threading model for rendering

[RandomShaper]

The "single-unsafe" thread model for rendering wasn't actually honored in Godot 4, as there's no way it can work despite user's best efforts. Therefore, this PR finally removes the option, or rather deprecates it.

The project setting is remodeled so it looks like the counterpart settings for physics. The takeaway is that projects that were using separate-thread rendering need to change the setting again. It's still considered experimental, so I don't think that's a big issue as long as the fact is documented in the release notes.