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/helper/NotebookTab.cpp

@@ -89,7 +89,7 @@ NotebookTab::NotebookTab(Notebook *notebook)
 
     this->menu_.addAction(
         "Close Tab",
-        [=]() {
+        [this]() {
             this->notebook_->removePage(this->page);
         },
         getApp()->hotkeys->getDisplaySequence(HotkeyCategory::Window,
@@ -97,7 +97,7 @@ NotebookTab::NotebookTab(Notebook *notebook)
 
     this->menu_.addAction(
         "Popup Tab",
-        [=]() {
+        [this]() {
             if (auto container = dynamic_cast<SplitContainer *>(this->page))
             {
                 container->popup();