zfs_2_1: remove

With the release of 2.3.0-rc, we know that 2.3.0 will be coming sometime
soon. Per the [ZFS release policy][1], only the current and previous
releases are expected to be supported, so 2.1.x will become unsupported.
Unfortunately upstream does not have any specific timelines, so we do
not know when it will become unsupported, but when it does we will
likely backport the removal. As such, begin warning of imminent removal.

[1]: https://github.com/openzfs/zfs/blob/6187b194349c5a728c9df8c6842f1866d5c9782a/RELEASES.md

nixos/tests/zfs.nix

@@ -191,12 +191,6 @@ let
 
 in {
 
-  # maintainer: @raitobezarius
-  series_2_1 = makeZfsTest {
-    zfsPackage = pkgs.zfs_2_1;
-    kernelPackages = pkgs.linuxPackages;
-  };
-
   series_2_2 = makeZfsTest {
     zfsPackage = pkgs.zfs_2_2;
     kernelPackages = pkgs.linuxPackages;

pkgs/top-level/all-packages.nix

@@ -12924,17 +12924,13 @@ with pkgs;
 
   inherit
     ({
-      zfs_2_1 = callPackage ../os-specific/linux/zfs/2_1.nix {
-        configFile = "user";
-      };
       zfs_2_2 = callPackage ../os-specific/linux/zfs/2_2.nix {
         configFile = "user";
       };
       zfs_unstable = callPackage ../os-specific/linux/zfs/unstable.nix {
         configFile = "user";
       };
     })
-    zfs_2_1
     zfs_2_2
     zfs_unstable;
   zfs = zfs_2_2;

pkgs/top-level/linux-kernels.nix

@@ -578,10 +578,6 @@ in {
 
     zenpower = callPackage ../os-specific/linux/zenpower { };
 
-    zfs_2_1 = callPackage ../os-specific/linux/zfs/2_1.nix {
-      configFile = "kernel";
-      inherit pkgs kernel;
-    };
     zfs_2_2 = callPackage ../os-specific/linux/zfs/2_2.nix {
       configFile = "kernel";
       inherit pkgs kernel;