Should switch to require >= C++17

[EricCousineau-TRI]

We gained official support via #11610, and have been using C++17 in Anzu, and it's been great.
We should eventually require C++17, esp. once we drop Xenial (#10606),

Strawman date to require >= C++17: 2019-11-15 ? 2019-10-25 (same as timeline issue)

\cc @jwnimmer-tri @jamiesnape

[jwnimmer-tri]

Yes, as soon as Xenial and High Sierra are gone, we should make -std=c++17 our minimum (and our default).

[jamiesnape]

So I think you can probably just say that the last day we guarantee that a non-C++17 compiler will work is the day we remove Xenial. We don't need a different date per se.