Compilation failure with C++14

[jmr]

As reported in #400 (comment) there is a compilation failure with C++14 due to if constexpr.

/s2geometry/src/s2/util/math/vector.h:103:8: warning: ‘if constexpr’ only available with ‘-std=c++17’ or ‘-std=gnu++17’ [-Wc++17-extensions]
  103 |     if constexpr (I >= 0 && I < SIZE) return AsD().Data()[I];

We can either rewrite to avoid if constexpr or drop C++14 support.