Build as C++20

Switch CMake (including the Core and GPU compile features and the Android
flag), the Visual Studio projects (stdcpp20), the legacy Android NDK
makefiles, the libretro makefiles and .ycm_extra_conf.py from C++17 to
C++20.

[=] lambdas that use this implicitly become [=, this], to avoid
-Wdeprecated-this-capture.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Common/UI/ViewGroup.cpp

@@ -1121,7 +1121,7 @@ CollapsibleSection::CollapsibleSection(std::string_view title, LayoutParams *lay
 	SetSpacing(0.0f);
 
 	header_ = Add(new CollapsibleHeader(open_, title));
-	header_->OnClick.Add([=](UI::EventParams &) {
+	header_->OnClick.Add([=, this](UI::EventParams &) {
 		// Change the visibility of all children except the first one.
 		// Later maybe try something more ambitious.
 		UpdateVisibility();