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/AccountSwitchWidget.cpp

@@ -20,7 +20,7 @@ AccountSwitchWidget::AccountSwitchWidget(QWidget *parent)
         this->addItem(userName);
     }
 
-    app->accounts->twitch.userListUpdated.connect([=]() {
+    app->accounts->twitch.userListUpdated.connect([=, this]() {
         this->blockSignals(true);
 
         this->clear();
@@ -39,7 +39,7 @@ AccountSwitchWidget::AccountSwitchWidget(QWidget *parent)
 
     this->refreshSelection();
 
-    QObject::connect(this, &QListWidget::clicked, [=] {
+    QObject::connect(this, &QListWidget::clicked, [=, this] {
         if (!this->selectedItems().isEmpty())
         {
             QString newUsername = this->currentItem()->text();