WarnManager.cpp: simplify an expression using std::format (C++20 feature)

[lucafedeli88]

C++20 has introduced a [text formatting library](see https://en.cppreference.com/cpp/utility/format) that greatly simplifies certain expressions.

This PR exemplifies this new feature by changing the expression :

                "["
                + std::string(abl_msg_logger::PriorityToString(msg_priority))
                + "]["
                + topic
                + "] "
                + text));

into

                std::format("[{}][{}] {}",
                    abl_msg_logger::PriorityToString(msg_priority), topic, text)));

I've verified that, after this substitution, the warning logger produces a text message identical to the one of the test presented here: https://warpx.readthedocs.io/en/latest/developers/warning_logger.html .

If no issues are reported, I would like to expand the usage of std::format in WarpX.

[lucafedeli88]

I am closing this, because one of the compiler combinations that we want to support (nvcc with gcc v11.4.0 as the host compiler) does not support std::format.