Add debian 11 bullseye official support

For the haskell docker images [we want to support debian 11 bullseye](https://github.com/haskell/docker-haskell/issues/73). In terms of GHC, cabal and stack's debian 11 support the state of play is:

* GHC x86_64 - Support as of 9.4.1
* GHC aarch64 - [Support coming in ~~9.4.3~~ 9.6.1](https://gitlab.haskell.org/ghc/ghc/-/issues/22005)
* stack x86_64 - Appears to work for 2.7.5
* stack aarch64 - [stack aarch64 is not being actively supported](https://github.com/haskell/docker-haskell/issues/59), so is excluded from the deb10 (and deb11) images which is fine.

Therefore, once GHC ~~9.4.3~~ 9.6.1 (it has been postponed apparently) comes out we are good to add 9.6 line support for debian 11 excluding cabal.

cabal has support for `x86_64-linux-deb10` and `aarch64-linux-deb10`, but does not include a release targeting debian 11. I have done some [basic testing](https://github.com/haskell/docker-haskell/pull/88) using `cabal-install-3.8.1.0-x86_64-linux-deb10` in debian 11 and it appears to work. I have not tested `cabal-install-3.8.1.0-aarch64-linux-deb10`.

Can cabal add official support for debian 11?