fix: remove deprecated operator"" syntax usage (#661)

Clang warns about this. Apparently GCC 4.9 may warn about the fixed
syntax, though!

Spotted in the ADBC CI:

```
 /adbc/c/vendor/nanoarrow/nanoarrow.hpp:94:35: error: identifier '_asv' preceded by whitespace in a literal operator declaration is deprecated [-Werror,-Wdeprecated-literal-operator]
   94 | inline ArrowStringView operator"" _asv(const char* data, std::size_t size_bytes) {
      |                        ~~~~~~~~~~~^~~~
      |                        operator""_asv
```

src/nanoarrow/nanoarrow.hpp

@@ -92,9 +92,16 @@ namespace literals {
 /// @{
 
 /// \brief User literal operator allowing ArrowStringView construction like "str"_asv
+#if !defined(__clang__) && (defined(__GNUC__) && __GNUC__ < 6)
 inline ArrowStringView operator"" _asv(const char* data, std::size_t size_bytes) {
   return {data, static_cast<int64_t>(size_bytes)};
 }
+#else
+inline ArrowStringView operator""_asv(const char* data, std::size_t size_bytes) {
+  return {data, static_cast<int64_t>(size_bytes)};
+}
+#endif
+// N.B. older GCC requires the space above, newer Clang forbids the space
 
 // @}