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>

.github/workflows/ubuntu.yml

@@ -362,13 +362,11 @@ jobs:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
-      # The module test uses `import std;`, which needs CMake's experimental
-      # import-std support. Its opt-in token is CMake-version-specific, so pin
-      # CMake to the version whose token is set in tests/module_cpp20/CMakeLists.txt.
-      - name: Get pinned CMake and ninja
+      # The module test uses `import std;`, which needs CMake 4.5 or newer.
+      - name: Get CMake 4.5+ and ninja
         uses: lukka/get-cmake@f86dd60acae0d132056913e248bdabf0e0580b7a # v4.4.5
         with:
-          cmakeVersion: 4.3.4
+          cmakeVersion: '^4.5.0'
       # Clang: the std library module is provided by libc++ (the image's libstdc++
       # ships none), and the image's libc++ module manifest has a broken relative
       # path — repoint it at the real module sources.