folly/Conv: remove double-conversion, replace toAppend<float> with fmt

[r-barnes]

Summary:
Removes the last remaining use of Google's double-conversion library from folly/Conv.h:

• Removes DtoaMode, DtoaFlags, their bitwise operators, and the detail:: helpers (convert(), kConvMaxFixedDigitsAfterPoint, kConvMaxPrecisionDigits, and the range constants). These were only needed to bridge toAppend(Src, Tgt*, DtoaMode, ...) to double-conversion; all callers have migrated away (folly/json in the previous diff, thrift/JsonWriter in the diff before this).
• Removes toAppend(Src, Tgt*, DtoaMode, unsigned, DtoaFlags) — the parametric overload is now dead.
• Replaces toAppend(Src value, Tgt* result) (the simple double-to-string overload) with fmt's Dragonbox. NaN and Infinity preserve the "NaN"/"Infinity"/"-Infinity" spellings. Other values use fmt::format("{}", ...). The output is the shortest round-trip decimal with fmt's exponent threshold; a small number of testDoubleToString golden values change accordingly (e.g., "0.000001" → "1e-06", uppercase "E" → lowercase "e") since fmt and double-conversion use different formatting conventions.
• Simplifies estimateSpaceNeeded<float> to a constant 24 + sign, removing dependence on the removed detail constants.
• In fbcode/folly/BUCK: swaps exported_external_deps = ["double_conversion"] for "fbsource//third-party/fmt:fmt" in the :conv target.
• Removes the DtoaMode/DtoaFlags ConvTest tests that tested the removed API.

Reviewed By: skrueger

Differential Revision: D106051019

[meta-codesync]

@r-barnes has exported this pull request. If you are a Meta employee, you can view the originating Diff in D106051019.