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>

Core/Debugger/WebSocket/GPURecordSubscriber.cpp

@@ -72,7 +72,7 @@ void WebSocketGPURecordState::Dump(DebuggerRequest &req) {
 		haveGPU = PSP_GetBootState() == BootState::Complete && gpu != nullptr;
 		if (!haveGPU)
 			return;
-		started = gpu->GetRecorder()->RecordNextFrame([=](const Path &filename) {
+		started = gpu->GetRecorder()->RecordNextFrame([=, this](const Path &filename) {
 			lastFilename_ = filename;
 			pending_ = false;
 		});