Update C++ linter to use gcc default version of C++

[jxu]

Now that C++17 has been out for a while, I think it is a better default for the C++ linter to not force C++14 and instead use the system default. I'm on Ubuntu 22.04 and the system default C++ version is now c++17 (actually gnu++17).

https://github.com/zyedidia/micro/blob/2898f1590d7a0588c5ae27753b2d68bd2607156a/runtime/plugins/linter/linter.lua#L69

I can submit a PR assuming it is just removing the flag from linter.lua.

[Andriamanitra]

I think that would be good change, at this point defaulting to c++14 is just weird. The other option to consider would be to default to the latest C++ version so it at least never complains about valid code (because of backwards compatibility guarantees), but I think using the system default is more sensible as it doesn't need to be updated every time there's a new C++ version.

If users want to specify a certain C++ version they can add makeLinter() in their init.lua.

[JoeKar]

This change was already recommended in #2962 too (see this comment).

[jxu]

Maybe if this PR is small it can be accepted quicker.