doc/rl-2505: Add deprecation warning for removal of GHC versions.

Author: maralorn

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

@@ -17,6 +17,7 @@
 
 - The default GHC version has been updated from 9.6 to 9.8.
   `haskellPackages` also uses Stackage LTS 23 (instead of LTS 22) as a baseline.
+  We aim to remove the old GHC versions 8.10, 9.0 and 9.2 in the next release in accordance with [the new GHC deprecation policy](https://discourse.nixos.org/t/nixpkgs-ghc-deprecation-policy-user-feedback-necessary/64153).
 
 - LLVM has been updated from LLVM 16 (on Darwin) and LLVM 18 (on other platforms) to LLVM 19.
   This introduces some backwards‐incompatible changes; see the [upstream release notes](https://releases.llvm.org/) for details.