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/GeneralPageView.cpp

@@ -39,7 +39,7 @@ GeneralPageView::GeneralPageView(QWidget *parent)
         {scrollArea, new QSpacerItem(16, 1), navigation}));
 
     QObject::connect(scrollArea->verticalScrollBar(), &QScrollBar::valueChanged,
-                     this, [=] {
+                     this, [this] {
                          this->updateNavigationHighlighting();
                      });
 }
@@ -74,7 +74,7 @@ TitleLabel *GeneralPageView::addTitle(const QString &title)
     navLabel->setCursor(Qt::PointingHandCursor);
     this->navigationLayout_->addWidget(navLabel);
 
-    QObject::connect(navLabel, &NavigationLabel::leftMouseUp, label, [=] {
+    QObject::connect(navLabel, &NavigationLabel::leftMouseUp, label, [=, this] {
         this->contentScrollArea_->verticalScrollBar()->setValue(label->y());
     });
 

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)"));

src/widgets/settingspages/ModerationPage.cpp

@@ -213,7 +213,7 @@ void ModerationPage::addModerationButtonSettings(
     texts->setContentsMargins(0, 0, 0, 15);
     texts->setSizeConstraint(QLayout::SetMaximumSize);
 
-    const auto valueChanged = [=] {
+    const auto valueChanged = [=, this] {
         const auto index = QObject::sender()->objectName().toInt();
 
         const auto line = this->durationInputs_[index];