Make C++17 the minimum supported standard

[NAThompson]

The CI tasks now require at least 3 hours of latency and are arguably at least 30 hours of raw compute. This is pressing the bounds of common decency against the free tier of CI. I think it needs to be cut down.

Proposal: Remove support for all C++ standards below C++17.

[mborland]

I concur. In the next release we could state it's the last to support C++14. C++17 would be a decade old by the time our first release with it as the minimum hits.

[ckormanyos]

Would it make sense to try and reduce the depth of testing in addition, or as a potential option, to potentially dropping a language standard?

That would be more challenging, but might be worth thinking about. I suspect, but don't know, that some of the runs in debug mode might be the long ones. But I've never taken a serious look into this matter previously.