Changes

Require C++17. (#1225)

* Require C++17.

  - This is an ambitious but reasonable change. I am assuming OMSimulator
    does not mind supporting only the systems which OpenModelica has support for.
    In which case, the oldest `gcc` version we are required to support is
    version 8.5. `gcc-8.5` should have support for C++17. More specifically
    it should have support for the <filesystem> library which OMSimulator
    needs.

* Remove CentOS-7 PR check.

  - We do not intend to support `CentOS-7` any longer. I think it was just
    left as it was because it never broke.

* Use the more recent docker image for testing Linux builds.

  - The `v1.16.3` image comes with `gcc-7.5`. the minimum we are required to
    support is `gcc-8.5`. Normally this would be fine but `gcc-7.5` has only
    experimental support for the `C++17` `<filesystem>` library we need.

* Remove uses and references to `boost::filesystem`.