Redundant CMAKE_CXX_STANDARD 17

[flowerthrower]

Review whether any targets still rely on the global CMAKE_CXX_STANDARD

This should be explored in a separate PR. In principle I'd argue that this should not be necessary. the MQT Core libraries have a target compile feature definition for the C++ standard and that should be propagated.

Originally posted by @burgholzer in #881 (comment)

[burgholzer]

We no longer use CMAKE_CXX_STANDARD for the core libraries here.
MLIR still uses quite some global settings, which is why the respective setting of the C++ standard is here to stay.
However, it is now set as a Cache variable, which means that it can be overridden from the outside, e.g., to raise the required standard to C++20.
See

core/mlir/CMakeLists.txt

Lines 9 to 11 in 706ece8

	set(CMAKE_CXX_STANDARD
	17
	CACHE STRING "C++ standard to conform to")