chore: upgrade to phoenix_html 4

[nwittstruck]

Further Notes

From the phoenix_html 4 changelog:

This version removes deprecated functionality and moved all HTML helpers to a separate library. HTML Helpers are no longer used in new apps from Phoenix v1.7. Older applications who wish to maintain compatibility, add {:phoenix_html_helpers, "~> 1.0"} to your mix.exs and then replace use Phoenix.HTML in your applications by:

Possible Implementation

This application has been started with phoenix 1.6. We should therefore upgrade to phoenix_html 4.

Checklist

• Upgrade to phoenix_html 4
• Upgrade live view to 0.20.x or (if available by then) 1.x
• Add tests