[Rework] Switch minimum C++ standard version to C++20

CMakeLists.txt

 
 # This is supported merely with cmake 3.1
 SET(CMAKE_C_STANDARD 11)
-SET(CMAKE_CXX_STANDARD 17)
+SET(CMAKE_CXX_STANDARD 20)
 SET(CMAKE_C_STANDARD_REQUIRED ON)
 SET(CMAKE_CXX_STANDARD_REQUIRED ON)
 LIST(APPEND CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/")

src/libserver/html/html.cxx

 
 		if (c == '\0') {
 			/* Replace with u0FFD */
-			parser_env.buf.append(u8"\uFFFD");
+			parser_env.buf.append((const char *)u8"\uFFFD");
 		}
 		else {
 			parser_env.buf.push_back(c);
 				const auto last = input.cend();
 				for (auto it = input.cbegin(); it != last; ++it) {
 					if (*it == '\0') {
-						output.append(u8"\uFFFD");
+						output.append((const char *)u8"\uFFFD");
 					}
 					else {
 						output.push_back(*it);