GH-47881: [C++] Update fast_float version to 8.1.0

Besides bumping to a much newer version of this vendored library, this also fixes https://issues.oss-fuzz.com/issues/452080278 (found by our CSV reader fuzzer).

Author: pitrou

cpp/src/arrow/util/value_parsing_test.cc

@@ -147,6 +147,12 @@ TEST(StringConversion, ToFloat) {
   AssertConversionFails<FloatType>("");
   AssertConversionFails<FloatType>("e");
   AssertConversionFails<FloatType>("1,5");
+  AssertConversionFails<FloatType>("-");
+  AssertConversionFails<FloatType>("+");
+  AssertConversionFails<FloatType>("-+");
+  AssertConversionFails<FloatType>("+-");
+  AssertConversionFails<FloatType>("--0");
+  AssertConversionFails<FloatType>("++0");
 
   StringConverter<FloatType> converter(/*decimal_point=*/',');
   AssertConversion(&converter, "1,5", 1.5f);

cpp/src/arrow/vendored/fast_float/README.md

@@ -20,7 +20,7 @@
 # fast_float
 
 The files in this directory are vendored from fast_float
-git tag `v3.10.1`.
+git tag `v8.1.0`.
 
 See https://github.com/fastfloat/fast_float
 
@@ -30,10 +30,10 @@ See https://github.com/fastfloat/fast_float
 
 ## How to update
 
-You must replace `VERSION` in the command lines with suitable version
-such as `3.10.1`.
+You must replace `VERSION` in the command line below with suitable version
+such as `v8.1.0`.
 
 ```bash
-cpp/src/arrow/vendoered/fast_float/update.sh VERSION
+cpp/src/arrow/vendored/fast_float/update.sh VERSION
 git commit add cpp/src/arrow/vendoered/fast_float/
 ```