doc/rl-2505: relnote Plasma 5 deprecation

Author: K900

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

@@ -57,3 +57,5 @@
   - `functor.wrapped` is now deprecated for some types and using it will give a warning with migration instructions. It is deprecated for these types:
     - `lib.types.attrsWith`
     - `lib.types.listOf`
+
+- Plasma 5 and Qt 5 based versions of associated software are deprecated in NixOS 25.05, and will be removed in NixOS 25.11. Users are encouraged to upgrade to Plasma 6.