Update config to match the latest conventions

[grossvogel]

Recent elixir versions use a config scheme that's easier to understand, with a clear separation between compile-time and runtime configutation: https://hexdocs.pm/mix/Mix.Tasks.Release.html#module-runtime-configuration

We should update the template to match this setup so onboarding engineers can find things where they expect and can rely on the latest elixir docs for guidance.

[ijadams]

Have some SLA time this week going to pick this up and bundle it into another PR if everyone is okay with that?

[ijadams]

Also a candidate for @pjdiiori to pick up with his SLA shadow hours

[pjdiiori]

Is this just as simple as renaming "releases.exs" to "runtime.exs"?

I've found some examples that place all the configurations within a if config_env() == :prod do block but I don't see any mention of that in the documentation. Is that preferred/necessary?

[grossvogel]

I just realized something about this:

• releases.exs is only used for built releases
• runtime.exs is evaluated at runtime for all environments meaning it includes test and dev as well

So we have to be careful here. We probably have some compile-time config for test and dev that we don't want to overwrite with environment variables even though we do want to use environment for those configs in prod.

Some resources:

• https://www.youtube.com/watch?v=guc16D_0Imk
• https://hexdocs.pm/elixir/Config.html#import_config/1