Use CMake 4.5's stable import std support in the module test (#6103)

* Use CMake 4.5's stable import std support in the module test

CMake 4.5 makes `import std` available to every target that scans for
C++ modules and drops the experimental CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
gate and the CXX_MODULE_STD property. The module test no longer needs the
version-specific opt-in token, so the ci_module_cpp20 jobs no longer have
to pin CMake 4.3.4 and now request CMake ^4.5.0.

The modules, CMake and README documentation now list the CMake, generator
and toolchain requirements for building the module and for `import std;`.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Bump lukka/get-cmake from 4.4.5 to 4.5.0

get-cmake 4.4.5 does not know CMake 4.5.0, so the ci_module_cpp20 jobs
that request '^4.5.0' failed in the setup step. Bump every pin so all
jobs use the same action version.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Install unzip in the Clang module job before get-cmake

The ^4.5.0 CMake/Ninja cache key is new, so get-cmake has to unzip the
Ninja download, and silkeh/clang:latest has no unzip.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Disable C++ module scanning for the unit tests

With CMake 4.5, every target that scans for C++ modules gets a synthesized
std module target. For the C++20 and newer unit-test targets, this compiled
libstdc++'s std.cc with the -Werror test flags and failed to configure with
libc++ images that lack std.compat.cppm. The unit tests use no modules, so
turn scanning off; the module test is a separate project.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/module_cpp20/main.cpp

@@ -17,8 +17,8 @@ using namespace nlohmann::literals;
 // Standard-library facilities are pulled in via `import std;` rather than
 // textual `#include`s: mixing `import` with textual standard headers does not
 // compile under GCC's C++20 modules implementation, whereas `import std;` works
-// across GCC, Clang, and MSVC. This requires C++23 and CMake's (experimental)
-// import-std support; see CMakeLists.txt.
+// across GCC, Clang, and MSVC. This requires C++23 and CMake 4.5 or newer;
+// see CMakeLists.txt.
 int main()
 {
     // basic_json / json: parsing and value access