Fix compatibility with fmt 12.2.0

[xiaozhuai]

Problem

fmtlib/fmt deprecated <fmt/core.h> in commit 0007426. Starting from fmt 12.2.0, using <fmt/core.h> causes compilation errors because the deprecated include is now opt-in only.

Fix

Replace all #include <fmt/core.h> with #include <fmt/format.h>, which is the standard header in fmt 12.2.0+.

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[zhjwpku]

Since folly is now targeting c++20, why not just replace fmt with std::format?

[xiaozhuai]

Since folly is now targeting c++20, why not just replace fmt with std::format?

That's another matter.