Thanks for the pointer. I had to manually update the dependency versions in mix.exs, mix hex.outdated gave me what I pasted below. After I did that and ran mix deps.update --all it cleared up.
$ mix hex.outdated
Dependency Current Latest Update possible
ecto_sql 3.5.4 3.5.4
floki 0.30.0 0.30.0
gettext 0.18.2 0.18.2
jason 1.2.2 1.2.2
phoenix 1.5.8 1.5.8
phoenix_ecto 4.2.1 4.2.1
phoenix_html 2.14.3 2.14.3
phoenix_live_dashboard 0.2.0 0.4.0 No
phoenix_live_reload 1.3.0 1.3.0
phoenix_live_view 0.12.1 0.15.4 No
plug_cowboy 2.4.1 2.4.1
postgrex 0.15.8 0.15.8
telemetry_metrics 0.4.2 0.6.0 No
telemetry_poller 0.5.1 0.5.1