Remove clang11Stdenv

Clang 11 doesn't have support for three-way-comparisons (<=>, "spaceship
operator", "consistent comparisons") and is older than `clangStdenv`.

`clangStdenv` is currently 12 on FreeBSD and Android and 16 on other
platforms:

https://github.com/NixOS/nixpkgs/blob/32e718f00c26c811be0062dd0777066f02406940/pkgs/top-level/all-packages.nix#L16629-L16644

Let's start by removing Clang 11 from our distribution. Next we can
consider upgrading to Clang 17, which fully supports the spaceship
operator:

https://releases.llvm.org/17.0.1/tools/clang/docs/ReleaseNotes.html#what-s-new-in-clang-release

flake.nix

@@ -52,7 +52,6 @@
 
       stdenvs = [
         "ccacheStdenv"
-        "clang11Stdenv"
         "clangStdenv"
         "gccStdenv"
         "libcxxStdenv"