ICU-22920 Avoid CTAD in Formattable's constructor. NFC

[Quuxplusone]

This line uses CTAD on pair, when every other place in the codebase uses a function call to make_pair (and no other place uses CTAD on any class template at all). Assume this was unintentional, and fix it.

warning: class template argument deduction is incompatible with
C++ standards before C++17; for compatibility, use explicit type
name 'std::pair<const Formattable *, int>'
(aka 'pair<const icu_77::message2::Formattable *, int>') [-Wctad]
      Formattable(const Formattable* arr, int32_t len) : contents(std::pair(arr, len)) {}
                                                                  ^~~~~~~~~

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