Migrate to C++ 20 & switch to websocketpp develop branch (#4252)

* feat: c++ 20

* fix: c++ 20 deprecations

* fix(msvc): warnings

* chore: add changelog entry

* fix: formatting

* Update websocketpp to the `develop` branch

* Specify other template type in FlagsEnum != operator

* Remove the user of simple template ids in our websocketpp template class

Also standardizes the file a bit by using nested namespaces, using
pragma once

* fix: turn `MAGIC_MESSAGE_SUFFIX` into a `QString`

* hacky unhacky hacky const char hack

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/messages/layouts/MessageLayout.cpp

@@ -273,7 +273,7 @@ void MessageLayout::paint(QPainter &painter, int width, int y, int messageIndex,
     if (isLastReadMessage)
     {
         QColor color;
-        if (getSettings()->lastMessageColor != "")
+        if (getSettings()->lastMessageColor != QStringLiteral(""))
         {
             color = QColor(getSettings()->lastMessageColor.getValue());
         }