Bumping BGL's minimum C++ standard: input wanted on target version

[Becheler]

Hi Graphlings! 🚀

During our modernization workshop in Paris (see #466), one of the most frequent discussion points was the C++ standard version BGL should require. Modernization is intuitively at odds with C++14 😉

Some context worth knowing:

• Boost.Graph is already C++14
• A lot of API modernization can be done without bumping the minimum.
• Some modernization changes would land cleaner on a newer standard
• We don't yet have a clear enough picture of our users' constraint space to inform our discussion about a potential baseline bump. For example, it appeared that automotive industry users will be locked in C++17 for a while with MISRA.

Your answers will inform our discussion about a potential baseline bump.

We'd deeply appreciate it if you could take a moment to answer this simple poll. Thanks!