mkBinaryCache: add release notes entry about new zstd compression by default (#386506)

Author: flokli

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -274,6 +274,8 @@
   word-breaking. So you want to write `extraArgs = [ "--prefer" "spaced pat" ]`
   rather than previous `extraArgs = [ "--prefer 'spaced pat'" ]`.
 
+- `mkBinaryCache` now defaults to using `zstd` compression for the binary caches it creates. The previous `xz` compression method can be used by passing `compression = "xz";`.
+
 - `nodePackages."@commitlint/config-conventional"` has been removed, as it is a library, and projects should depend on it instead.
 
 -  zigbee2mqtt is now available in version 2.x as `zigbee2mqtt_2`. In NixOS 25.11 we'll remove `zigbee2mqtt_1` and default to `zigbee2mqtt_2`. See the [breaking changes](https://github.com/Koenkk/zigbee2mqtt/discussions/24198) announcement for 2.0.0.