Update Readme With Updated Default Versions

Hello, I'm just getting started with Elixir, Phoenix, and Heroku. I want to create an issue about updating the readme section where it lists how to use .config files. I gave up on Elixir/Phoenix and Heroku at one point, but I wouldn't have if the readme had pinned versions that worked the first time.

Backstory...I'd like to use Gigalixir to fully utilize the features, but I'm working within an enterprise contract, as may a lot of users using these buildpacks. I liked seeing the Heroku guide in the deployment section. https://hexdocs.pm/phoenix/heroku.html#content

However, I couldn't get Phoenix working by following that guide, which mentions these buildpacks. Initially, I listed my current local versions in my config files.

# phoenix_static_buildpack.config
node_version=14.16.0
npm_version=6.14.11

# elixir_buildpack.config
erlang_version=23.0
elixir_version=1.11.1

I won't go through the errors, but I quickly gave up and started checking out Rails, since I see how prevalent it is on Heroku. Not much trouble with Rails on Heroku, but then I got to not having hot reloading setup by default...and I went back to see if I could fix my issues.

Long story short...it took me several tries to get versions pinned that will work for my setup.

# phoenix_static_buildpack.config
node_version=14.16.0
npm_version=6.14.11

# elixir_buildpack.config
erlang_version=22.3
elixir_version=1.9.0

So, my proposal is to definitely update the readme file as time goes along with whatever the best versions are for Elixir + Phoenix, as I think a lot of people will be using Phoenix and it's the largest draw to Elixir, in my brief experience.

With GitHub's dependency tracking, is it possible to add a config file to this repo that would create issues with Dependabot or similar every time any language dependency updates? Then update the readme...and/or defaults in the config files in the PR.

Or maybe a GitHub Actions test running on cron once a day that attempts to install a basic elixir project with separate .config files tied to the versions in the readme? If it fails people get notified to update something.

I just want to keep the readme up-to-date, as I think anyone starting out looks to that place in the readme first...especially if they see the listed versions as being out-of-date.

I'll stop rambling...and I'm sure I'm missing things in the readme, but I think like 90% of devs, I scrolled to https://github.com/HashNuke/heroku-buildpack-elixir#configuration and see that as the most important section of the readme.

I also have some notes on https://hexdocs.pm/phoenix/heroku.html#content, but I'm not sure who controls that or how to update it.

Start with the README's configuration section and the phoenix_static_buildpack.config and elixir_buildpack.config examples described in the issue. Determine which pinned versions should be documented, then update the README and verify that the examples remain consistent; the proposed dependency tracking or scheduled test would require a separate scope decision.