Chore: Comprehensively Update this Tutorial to `Phoenix v1.7`

[Nelson]

With the recent release of Phoenix v1.7.0-rc.2 (which looks reasonably stable),
Now is a good time to comprehensively update this tutorial to the latest and greatest.
This is a great opportunity for us to see the differences between 1.6.X and 1.7 in a more involved project.

Todo

• git clone this project to your localhost and rename it to phx-chat-OLD
• use this "old" version as the basis for creating the new one, but ...
• git clone the project again and then DELETE all the code in the directories and mix.exs/mix.lock file:

• Create a new branch on your localhost e.g. phx-1.7-issue#X and
• Follow the instructions in the REAMDE.md to create a new app from scratch.
• Please remember to exclude all the noise that we don't need in a demo app like this e.g:

mix phx.new chat --no-mailer --no-dashboard --no-gettext

We really don't need all the noise from constant @dependabot updates for deps we aren't using in the example! ⏳

Note: this tutorial will NOT use LiveView.
We have a different one for that: https://github.com/dwyl/phoenix-liveview-chat-example
That is deliberate. We still use "MVC" (even though Phoenix has removed the "V" ... https://www.germanvelasco.com/blog/phoenix-1-7-is-view-less ... 🙄 )
So we want this tutorial to continue to showcase the M🙃C workflow even if we use LiveView in some other places.
I have zero intention of using LiveView for the basic flow in Auth for example. It's just not appropriate.

• Update the README.md as needed to match the updates in Phoenix.
• Match all the features in the current example including the Tailwind UI/UX and auth
  • update auth.md as needed (shouldn't be too much)
• Create a Pull Request with the update.
• Review your own PR before assigning it for review.
• Assign the PR to @SimonLab for review. 🙏

I expect this update to take a few hours. (T4h? 🤷‍♂️)
It pains me that we have to spend this time. I wish there weren't so many breaking changes! 😢
But the faster we can make this update the faster I (@nelsonic) can learn what I need to re-build auth dwyl/auth#207 🤞

Thanks! 🙏

[Nelson]

PR: #152