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:

32e718f00c/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

doc/manual/src/contributing/hacking.md

@@ -31,7 +31,7 @@ This shell also adds `./outputs/bin/nix` to your `$PATH` so you can run `nix` im
 To get a shell with one of the other [supported compilation environments](#compilation-environments):
 
 ```console
-$ nix develop .#native-clang11StdenvPackages
+$ nix develop .#native-clangStdenvPackages
 ```
 
 > **Note**
@@ -96,7 +96,7 @@ $ nix-shell
 To get a shell with one of the other [supported compilation environments](#compilation-environments):
 
 ```console
-$ nix-shell --attr devShells.x86_64-linux.native-clang11StdenvPackages
+$ nix-shell --attr devShells.x86_64-linux.native-clangStdenvPackages
 ```
 
 > **Note**

flake.nix

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