Harden JSON_HAS_RANGES detection for incomplete C++20 ranges implementations

[nlohmann]

Some standard libraries advertise __cpp_lib_ranges but ship an incomplete
C++20 ranges implementation. Constructing a basic_json from a view (e.g.
j | std::views::filter(...)) then asserts or fails to compile on those
toolchains.

This PR refines the JSON_HAS_RANGES feature detection to disable ranges
support on the affected standard libraries:

• libstdc++ < 11 (GCC 10) — assertion failure (closes Assert when using std::views::filter and GCC 10 #4440)
• libc++ < 16
• Clang < 16 combined with libstdc++ — compile error unless
  JSON_HAS_RANGES=0 was set manually (closes compiler error using clang-16.0.5 when using gcc-13.1 standard library #4051)

On these toolchains the library now falls back to JSON_HAS_RANGES 0
instead of enabling a broken code path. Toolchains with complete ranges
support are unaffected.

It also adds a regression test (unit-regression2.cpp) reproducing #4440
with std::views::filter, guarded by JSON_HAS_RANGES == 1 so it only runs
where ranges are actually available.

Closes #4440.
Closes #4051.

[github-actions]

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