Set index-state in generated cabal.config files

The `cabal.config` files generated by `stackage-server` are useful to replicate a Stackage snapshot using `cabal`. However currently they say "Due to revisions, this file may not work" and indeed this sometimes happens when package maintainers tighten bounds (e.g. https://github.com/snoyberg/http-client/pull/563#issuecomment-2754112813).

Cabal/Hackage now have a feature to address this, namely `index-state`. Would it be possible to have the `cabal.config` file include an `index-state` with the timestamp of the Hackage index used to build the snapshot? That would avoid the possibility of subsequent revisions breaking the snapshot (although it would mean users have to manually change the `index-state` to incorporate package updates not present at the time the snapshot was taken).

cc @sol