Fix potential compatibility with C++20 / C++23 mode

This fixes several issues with compatibility with C++20. C++23 was also
tested, but apparently it does not have any additional breaking changes
compared to C++20 (or we don't have those).

VCMI still uses C++17 as before - goal is only to make potential
transition easier.

There were 2 cases that are deprecated in C++20 that we use:
- Floating point operations on enums are deprecated
- `this` can no longer be captured when using default capture by value
`[=]`

Both of those should now be replaced with code that works fine in both C+
+17 and in C++20 mode

lib/texts/TextOperations.cpp

@@ -211,8 +211,12 @@ void TextOperations::trimRightUnicode(std::string & text, const size_t amount)
 
 size_t TextOperations::getUnicodeCharactersCount(const std::string & text)
 {
-	std::wstring_convert<std::codecvt_utf8<char32_t>, char32_t> conv;
-	return conv.from_bytes(text).size(); 
+	size_t charactersCount = 0;
+
+	for (size_t i=0; i<text.size(); i += getUnicodeCharacterSize(text[i]))
+		charactersCount++;
+
+	return charactersCount;
 }
 
 std::string TextOperations::escapeString(std::string input)