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/widgets/settingspages/HighlightingPage.cpp

@@ -269,7 +269,7 @@ HighlightingPage::HighlightingPage()
             auto selectFile = customSound.emplace<QPushButton>("Change...");
 
             QObject::connect(selectFile.getElement(), &QPushButton::clicked,
-                             this, [=]() mutable {
+                             this, [this]() mutable {
                                  auto fileName = QFileDialog::getOpenFileName(
                                      this, tr("Open Sound"), "",
                                      tr("Audio Files (*.mp3 *.wav)"));