[lang-kit] dev-lang/zig-0.16.0 requires LLVM 21.x

Nobody has claimed this yet.

```
loading initial cache file /var/tmp/portage/dev-lang/zig-0.16.0/work/zig-0.16.0/build/gentoo_common_config.cmake
-- The C compiler identification is Clang 20.1.8
-- The CXX compiler identification is Clang 20.1.8
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/lib/llvm/20/bin/x86_64-pc-linux-gnu-clang - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/lib/llvm/20/bin/x86_64-pc-linux-gnu-clang++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring zig version 0.16.0
CMake Error at cmake/Findllvm.cmake:30 (message):
expected LLVM 21.x but found 20.1.8 using /usr/lib/llvm/20/bin/llvm-config
Call Stack (most recent call first):
CMakeLists.txt:136 (find_package)
```
We revert the update for now and we keep 0.16.0 as `zig-bin`

Start with cmake/Findllvm.cmake and CMakeLists.txt:136, using the reported dev-lang/zig-0.16.0 build log. Check how the package handles the LLVM 21.x requirement when LLVM 20.1.8 is installed. Done means resolving that requirement or confirming why 0.16.0 must remain available only as zig-bin.

Written by the indexing model from the issue text.