Boost.Graph 1.95 will be C++17

[Becheler]

Dear Boost.Graph community,

In two release cycles (Boost release 1.95 in summer 2027) Boost.Graph will be bumped to C++17 😄

This decision follows two polls that did not surface any C++14 user base or demand:

• Bumping BGL's minimum C++ standard: input wanted on target version #490
• Reddit poll

This standard version bump will bring benefits for maintainance, API and dependencies reduction:

• if constexpr,
• constexpr lambdas,
• [[nodiscard]], [[maybe_unused]] ...
• std::any instead of boost::any
• std::invoke_result instead of boost::result_of

We will not be able to guarantee backward compatibility with C++14. Please contact us if you are stuck in C++14.