What version of OpenGL should I target for Steam?
All of them.
Jokes aside, there is no definite version to target and previous statement holds some truth. Although I would use ~GL3.0 as the minimum requirement, everything else is optional. This is quite simply, because you obviously can't take use extensions which are not supported while it would also be stupid not to take advantage of the hot new extensions. This requires quite a bit of coding of course because you need to check which extensions are supported and select different ways to acheive a specific goal using said extensions but imo its the way to go. And with some clever abstraction it's not that hard.
A good example of this is
GL_ARB_direct_state_access. Although support is still not that great with some odd 20%, but why wouldn't I take advantage of it on the platforms that do support it.
Around GL3.0 should be the minimum required version while still using optional extensions that are available.