Enable use of std::format for AppleClang / Xcode 16

Specified the minimum apple-clang version to support `std::format` to 16
for _connanfile.py_.  Xcode 16 does include the `<format>` header but
does not seem to have the __cpp_lib_format compatibility macro defined
so a similar override was needed as we had for Clang 17.

To be able to use `std::format` with Xcode 16 I had to pass
`-DMP_UNITS_API_STD_FORMAT=AUTO` when calling `cmake`.  Is this expected
or is this a sign I missed something in my changes to _CMakeLists.txt_?

conanfile.py

@@ -99,7 +99,7 @@ class MPUnitsConan(ConanFile):
                 "compiler": {
                     "gcc": "13",
                     "clang": "17",
-                    "apple-clang": "",
+                    "apple-clang": "16",
                     "msvc": "194",
                 },
             },