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>

README.md

@@ -165,7 +165,8 @@ std::ifstream f("example.json");
 json data = json::parse(f);
 ```
 
-If using modules (enabled with `NLOHMANN_JSON_BUILD_MODULES`), this example becomes:
+If using modules (enabled with `NLOHMANN_JSON_BUILD_MODULES`; `import std;` needs CMake 4.5 or newer, see the
+[modules documentation](https://json.nlohmann.me/features/modules/)), this example becomes:
 ```cpp
 import std;
 import nlohmann.json;