Add option system.stateVersion

This option requests compatibility with older NixOS releases with
respect to stateful data, in cases where new releases have defaults
that might be incompatible with system state of existing NixOS
deployments. For instance, if we change the default version of
PostgreSQL, existing deployments will break if the new version can't
read databases created by the old version.

So for example, setting

  system.stateVersion = "15.07";

requests that options like services.postgresql.package use defaults
corresponding to the 15.07 release branch. Note that
nixos-generate-config emits this option. (In the future, NixOps may
set system.stateVersion to the NixOS release in use when the machine
was created.)

See also #7939 for another motivating example.