k3s: drop 1.24 & 1.25 for 23.05

In-line with the policy described
[here](https://github.com/NixOS/nixpkgs/blob/30b82a186bc585872624a298a5169d1d237ce6a4/pkgs/applications/networking/cluster/k3s/README.md#versions-in-nixos-releases)
(xref NixOS#224483), drop versions of k3s that will not be supported for the
full duration of the NixOS release.

Since 22.11 has k3s 1.25, that means we must have k3s 1.26 at least.

Both k3s 1.24 and 1.25 will lose support before the 23.11 nixos release
goes out of support, so we should drop them. Respectively, 1.24 loses
support in July 2023, and 1.25 loses support in October 2023. NixOS is
supported through December 2023.

nixos/doc/manual/release-notes/rl-2305.section.md

@@ -338,6 +338,8 @@ In addition to numerous new and upgraded packages, this release has the followin
 
 - The `qlandkartegt` and `garmindev` packages were removed due to being unmaintained and insecure.
 
+- Only `k3s` version 1.26 is included. Users of the `k3s_1_24` or `k3s_1_25` packages should upgrade to use the `1.26` version of the package.
+
 - `go-ethereum` package has been updated to v1.11.5 and the `puppeth` command is no longer available as of v1.11.0.
 
 - The `pnpm` package has be updated to from version 7.29.1 to version 8.1.1 and Node.js 14 support has been discontinued (though, there are workarounds if Node.js 14 is still required)