Compilation error introduced since 6.18.3

Created by: labodj

Hi @bblanchon, I updated this library from 6.18.2 to 6.18.3 and I'm experiencing compilation errors, everything was fine in 6.18.2

I'm compiling using avr-gcc 11.1.0 with c++20 features for an Arduino mega 2560

These are the errors:

/ArduinoJson/src/ArduinoJson/Strings/Adapters/ConstRamStringAdapter.hpp:48:32: error: expected unqualified-id before 'const'
/ArduinoJson/src/ArduinoJson/Strings/Adapters/ConstRamStringAdapter.hpp:48:32: error: expected ')' before 'const'
/ArduinoJson/src/ArduinoJson/Strings/Adapters/ArduinoStringAdapter.hpp:48:37: error: expected unqualified-id before 'const'
/ArduinoJson/src/ArduinoJson/Strings/Adapters/ArduinoStringAdapter.hpp:48:37: error: expected ')' before 'const'

The first error is located here:

template <int N>
class StringAdapter<const char[N]> : public StringAdapter<const char*> {
 public:
  StringAdapter<const char[N]>(const char* s) : StringAdapter<const char*>(s) {}
};

The second one is located here:

template <>
class StringAdapter< ::StringSumHelper> : public StringAdapter< ::String> {
 public:
  StringAdapter< ::StringSumHelper>(const ::String& s)
      : StringAdapter< ::String>(s) {}
};

Since 6.18.2 the 2 files above have been modified with these commits: 5790f3c8 6632fa8d