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/Application.cpp

@@ -551,7 +551,7 @@ void Application::initPubSub()
 
     this->twitch->pubsub->start();
 
-    auto RequestModerationActions = [=]() {
+    auto RequestModerationActions = [this]() {
         this->twitch->pubsub->setAccount(
             getApp()->accounts->twitch.getCurrent());
         // TODO(pajlada): Unlisten to all authed topics instead of only
@@ -561,7 +561,7 @@ void Application::initPubSub()
     };
 
     this->accounts->twitch.currentUserChanged.connect(
-        [=] {
+        [this] {
             this->twitch->pubsub->unlistenAllModerationActions();
             this->twitch->pubsub->unlistenAutomod();
             this->twitch->pubsub->unlistenWhispers();