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/CMakeLists.txt

@@ -1,11 +1,6 @@
-cmake_minimum_required(VERSION 3.30)
-
-# `import std;` (used by main.cpp) needs CMake's experimental C++ import-std
-# support. The accepted token is CMake-version-specific; this value matches the
-# CMake version pinned in the ci_module_cpp20 CI jobs (4.3.x). If that pin is
-# bumped, this token must be updated to match (configuration fails loudly
-# otherwise).
-set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "451f2fe2-a8a2-47c3-bc32-94786d8fc91b")
+# `import std;` (used by main.cpp) is supported without experimental opt-in
+# since CMake 4.5: it is available to every target that scans for C++ modules.
+cmake_minimum_required(VERSION 4.5)
 
 project(json_test CXX)
 
@@ -13,7 +8,6 @@ project(json_test CXX)
 set(CMAKE_CXX_STANDARD 23)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
-set(CMAKE_CXX_MODULE_STD ON)
 
 set(NLOHMANN_JSON_BUILD_MODULES ON CACHE BOOL "Enable nlohmann.json module support")