Replace phoenix_html dependency with phoenix_html_helpers

[tmaszk]

The latest version of phoenix_html package no longer includes <tag> and <content_tag> that phoenix_meta_tags uses. Those functions have been moved to phoenix_html_helpers. Also see changelog for recent changes.

This PR updates several things:

• use phoenix_html_helpers instead of phoenix_html
• use newer Config style (import Config vs use Mix.Config)
• update the CI unit test pipeline to include newer versions of elixir and OTP

NOTE: phoenix_html_helpers is dependent on phoenix_html version 4 as seen in the mix file here.

[NoBrainSkull]

You need to use PhoenixHTMLHelpers here :)