chore: update preview Plutus to 1.61 and CHaP flake input (#154)

Update CHaP flake input from 2025-10-23 to 2026-04-11 and
preview index-state to pick up plutus 1.61 from CHaP.

Closes #2159

Author: Unisay

cabal.project.preview

@@ -6,8 +6,8 @@
 
 import: cabal.project
 
--- Use latest CHaP index-state to access plutus-core 1.60.0.0
-index-state: cardano-haskell-packages 2026-03-30T00:00:00Z
+-- Use latest CHaP index-state to access plutus-core 1.61.0.0
+index-state: cardano-haskell-packages 2026-04-11T06:29:42Z
 
 -- Enable preview flag to build Preview.* modules with BuiltinCasing
 flags: +preview