release-notes/19.03: add release notes for nix 2.0 requirement bump

This is taken from the 18.09 change, which was reverted on release-18.09
but not master. The now-false 18.09 release notes were just removed from
master in 29854e24, but since the
underlying change is still there, release notes for 19.03 are warranted.
This commit takes the now-reverted release notes and reuses them for
that.