Update haskell version used by LeaseReport.

Part of PrivateStorageio#89 (closed)

Since I am working on moving ZKAPAuthorizer to use mach-nix, this would be the last remaining package using our nixpkgs fork. Since I was moving things around, I took some time to update the versions of everything this pins.

Add gitlab-ci configuration. (Part of privatestorage/privatestorageops#373)
Updates the haskell.nix version to the one used in https://github.com/PrivateStorageio/PaymentServer/pull/101
Updates the nixpkgs version from nixpkgs-20.03 to nixpkgs-21.05.
Updates that stackage LTS version to the most recent one supported by the pinned version of haskell.nix.
Use haskell-nix.project instead of haskell-nix.mkStackPkgSet, following the style used by PaymentServer.

Note: This MR was originally against a personal LeaseReport repository and approved by @jcalderone there. I had intended to transfer this repository to the PrivateStorage group after that MR was approved. However, it turns out that transferring requires either gitlab instance admin, or maintainer access to the group, but repository creation only requires developer access. Thus, I re-imported under the group, and created this MR, which is identical, except for the revert https://whetstone.privatestorage.io/privatestorage/LeaseReport/-/commit/72390c3ef884e65dd4d3cc7f07e456eea5c78780 of the redirect which was added to github.