Use C++20 in RemoteVstPlugin (#7916)

- Build RemoteVstPlugin in C++20 mode
- Avoids using std::wstring due to strange issues with it when built
with wineg++. See https://bugs.winehq.org/show_bug.cgi?id=58465
- Fix some memory leaks + minor cleanup of RemoteVstPlugin code
- Rename `F_OPEN_UTF8` to `fopenUtf8`
- Use C++20 in our `determine_version_from_source` CMake function
- Update ZynAddSubFX submodule

cmake/modules/ImportedTargetHelpers.cmake

@@ -44,7 +44,7 @@ function(determine_version_from_source _version_out _target _source)
 		_dvfs_run_result _dvfs_compile_result "${CMAKE_CURRENT_BINARY_DIR}"
 		SOURCES "${_source_file}"
 		LINK_LIBRARIES "${_target}"
-		CXX_STANDARD 17
+		CXX_STANDARD 20
 		RUN_OUTPUT_VARIABLE _run_output
 		COMPILE_OUTPUT_VARIABLE _compile_output
 	)