NixOS end-of-life instructions

[roberth]

I was expecting to find instructions for what to do when a NixOS release becomes unsupported.
I'd like to add instructions to update oldestSupportedRelease on master, which is a new value in NixOS/nixpkgs#163451. It improves the regular deprecation process.

[jonringer]

The only item is https://github.com/NixOS/release-wiki/blob/master/src/Release-Process-Walkthrough.md#a-month-after-release so far.

The linked PR looks more like nixos module change, rather than a consistent per-release step though.

[roberth]

Thanks! I've created #32 to support that nixpkgs PR.

The linked PR looks more like nixos module change, rather than a consistent per-release step though.

It does. The logic and docs are all about option renames, but it makes use of a variable that needs to be updated when the oldest release goes EOL, as that is the right time to start migrating out-of-tree modules.