php81: remove (#448348)

The different versions of PHP that nixpkgs provides are located under

attributes named based on major and minor version number; e.g.,

`php81` is PHP 8.1.

`php84` is PHP 8.4.

Only versions of PHP that are supported by upstream for the entirety

of a given NixOS release will be included in that release of

All available PHP attributes are wrappers around their respective

binary PHP package and provide commonly used extensions this way. The

real PHP 8.1 package, i.e. the unwrapped one, is available as

`php81.unwrapped`; see the next section for more details.

real PHP 8.4 package, i.e. the unwrapped one, is available as

`php84.unwrapped`; see the next section for more details.

Interactive tools built on PHP are put in `php.packages`; composer is

for example available at `php.packages.composer`.

- [sx](https://github.com/earnestly/sx), a simple alternative to both xinit and startx for starting a Xorg server. Available as [services.xserver.displayManager.sx](#opt-services.xserver.displayManager.sx.enable)

- [postfixadmin](https://postfixadmin.sourceforge.io/), a web based virtual user administration interface for Postfix mail servers. Available as [postfixadmin](#opt-services.postfixadmin.enable).

- [postfixadmin](https://postfixadmin.sourceforge.io/), a web based virtual user administration interface for Postfix mail servers.

- [prowlarr](https://wiki.servarr.com/prowlarr), an indexer manager/proxy built on the popular arr .net/reactjs base stack [services.prowlarr](#opt-services.prowlarr.enable).

- [input-remapper](https://github.com/sezanzeb/input-remapper), an easy to use tool to change the mapping of your input device buttons. Available at [services.input-remapper](#opt-services.input-remapper.enable).

- [InvoicePlane](https://invoiceplane.com), web application for managing and creating invoices. Available at [services.invoiceplane](#opt-services.invoiceplane.sites._name_.enable).

- [InvoicePlane](https://invoiceplane.com), web application for managing and creating invoices. Available at `services.invoiceplane`.

- [k3b](https://userbase.kde.org/K3b), the KDE disk burning application. Available as [programs.k3b](#opt-programs.k3b.enable).

  Note that system activation will complete before all certificates may have been renewed or acquired.

- `php81` was removed.

- `libvirt` now supports using `nftables` backend.

  - The `virtualisation.libvirtd.firewallBackend` option can be used to configure the firewall backend used by libvirtd.

  ./services/mail/pfix-srsd.nix

  ./services/mail/postfix-tlspol.nix

  ./services/mail/postfix.nix

  ./services/mail/postfixadmin.nix

  ./services/mail/postgrey.nix

  ./services/mail/postsrsd.nix

  ./services/mail/protonmail-bridge.nix

  ./services/web-apps/immich-public-proxy.nix

  ./services/web-apps/immich.nix

  ./services/web-apps/invidious.nix

  ./services/web-apps/invoiceplane.nix

  ./services/web-apps/isso.nix

  ./services/web-apps/jirafeau.nix

  ./services/web-apps/jitsi-meet.nix