nixos/release-notes: update release notes for COSMIC modules

<!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

- Initial support for the [COSMIC DE](https://system76.com/cosmic), a Rust-based desktop environment by System76, makers of Pop!_OS. Toggle the greeter (login manager) using `services.displayManager.cosmic-greeter.enable` and the DE itself with `services.desktopManager.cosmic.enable`. Mostly stable but still experimental. Please report any issues to the [COSMIC DE tracker in Nixpkgs](https://github.com/NixOS/nixpkgs/issues/259641) instead of upstream.

- NixOS now has initial support for the [**COSMIC DE**](https://system76.com/cosmic) which is currently at **Alpha 7**. COSMIC is a Rust-based Desktop Environment by System76, makers of Pop!_OS. Enable COSMIC by enabling the greeter (login manager) `services.displayManager.cosmic-greeter.enable` and the DE itself by enabling `services.desktopManager.cosmic.enable` options. The support in NixOS/nixpkgs is stable but still considered experimental because of the recent the addition. The COSMIC maintainers will be waiting for one more release of NixOS to determine if the experimental tag should be removed or not. Until then, please report any issues to the [COSMIC DE tracker in Nixpkgs](https://github.com/NixOS/nixpkgs/issues/259641) instead of upstream.

- `services.dex` now restarts upon changes to the `.environmentFile` or entries in `.settings.staticClients[].secretFile` when the entry is a `path` type.